Why can't I restore ver 4.0.100.1190 with a current backup?

I'm not sure I understand what you're aiming to do. If you want to be able to access the data in the old backup file, then you can open that backup in the current 7.6 version of Quicken and it will update to the current database format. It's still a separate file from your current "live" data file, and you can switch back and forth between your current file and the old file using File > Open Recent. (It may help to use File > Rename to give the old file a name which clearly identifies it, such as "My Quicken (OLD from 2017)". If you're switching between files often, I also recommend displaying the file name in the title bar: go to Settings > General and check the "Show file name in title bar" box.

Yes, I was questioning why. Since they have a current Quicken subscription, why try to open a backup file in Quicken 2017? Some piece of information is missing here.

For instance, is the user trying to move back to Quicken 2017 to cancel their current subscription and avoid ongoing subscription payments? If that's the case it might be possible to export the current data file to a QXF file, and then in Quicken 2017 create a new from-scratch file and import the data from the QXF file. Except I think Quicken 2017 needs user authorization to get running, and the server for authorization is no longer available, so I don't think it would work. But I'm making several guesses here, which is why I was asking @rcortesim for more information about why they're trying to use Quicken 2017 if they have a current subscription.

@rcortesim Sorry, I'm still a bit confused. You have Quicken on a Windows 7 computer. So that's Quicken Windows, correct? But you're talking about running Quicken 2017 for Mac, right? On which computer, the Mac? So is the Windows 7 computer irrelevant for this discussion? Both current Quicken Mac and the restored Quicken 2017 are on your Mac, right. I'm just trying to make sure I'm on the right page with you. 😀

If you are trying to save a backup from current subscription Quicken Mac, and then open it in Quicken Mac 2017, that just can't be done. The database format is constantly being updated, and you can't open a current Quicken Mac file in an older version, period. (It's different with Quicken windows, which hasn't changed the database format for more than a decade.) I'm still confused why, since you have a current subscription, you're trying to open a backup in a 7 year-old version.

No, the data file is not OS agnostic and neither is the backup file, I don't know where you got that. Quicken Mac cannot open a Quicken Windows file or vice versa. It is possible to move your data from one to a new file in the other but the conversion is not perfect. The files you get from the bank to import new account activity into Quicken are OS agnostic but that's not the same thing.

#2 may be true for Quicken Windows but it's not true for Quicken Mac.

@rcortesim The only thing I can suggest is trying what @jacobs suggested earlier - export your Quicken Mac data to a QXF file and then import that to Quicken Windows 2017.

I am running version 4.0.100.1190 (which I understand is 2017) on Windows 7. Last week, I subscribed to the latest Classic Deluxe for use on my newer MacBook. 
I have read in this forum that
1. The data file is OS agnostic. 
2. A backup from new versions of Quicken will restore older versions as far back as 2016. 
So, I’m confused why a backup from my new version will not be recognized by my 2017.

As @Jon wrote, your interpretation of what you've read is way off the mark. Regarding #1, I don't know where you thought you read that the data file is OS agnostic, as that's completely false; there are hundreds of threads on this forum about migrating data from Quicken Windows to Quicken Mac or vice versa. Regarding #2, I suspect what you've read about being able to use a current backup in an older version was regarding Quicken Windows. It is definitely not true for Quicken Mac.

I hope you can understand why there's been confusion in responding to your questions, because your original post — in a Quicken Mac category of the forum — was not clear that you were wanting to take a Quicken Mac version 7.6 backup and use it in Quicken Windows 2017. (Quicken Windows releases are usually denoted as "Rxx.xx", so your repeated reference to version 4.0.100.1190 wasn't clear you were referring to Quicken Windows; meanwhile, Quicken Mac 2017 was version 4.x — so it wasn't clear you were referring to trying to use the Mac backup file on Windows.)

It would still help to understand if you're trying to do this as just a one-time process of migrating your current data in Quicken Mac to the old Quicken Windows version, or as something you can do on a recurring basis. It sounds like you want to be able to regularly move from Quicken Mac to Quicken Windows? I'm not sure if you want to be able to move in the other direction as well?

In any case, we've established that the data files are not compatible. Moving data in either direction is not a matter of opening a data file, but going through a complex converter to build a new file. Things are lost in the conversion, including reports and attachments, settings, and more.

One additional key thing to know is that going from Quicken Mac to Quicken Windows does not work for investment accounts in the old Quicken 2017 version you are trying to use. That wasn't addressed until a 2023 update to Quicken Windows (R52.XX). So unless you're just dealing with banking accounts in Quicken, using Quicken's tool to convert a data file from Quicken Mac to Quicken Windows 2017 will not work.

@rcortesim I'm glad we finally got all the facts straight to so we got onto the same page. 😀 As a closing note, I'll mention two possible ways for you to have Quicken on your MacBook but access it from your old Windows computer…

If you want to enable Quicken's cloud syncing on your MacBook, you can specify which accounts to sync and then the past two years of your data will be uploaded into Quicken Cloud. (Going forward, more data will be added to the Cloud file.) Note that this does not require you to enable accounts for downloading from financial institutions; you can continue to enter everything manually. Then, on your Win 7 machine, you could use Quicken's web browser interface (aka "Quicken on the Web") to view and even enter data, independent of the MacBook.

There are some of us who are wary of Quicken Cloud syncing because of various ongoing reports from users about newer data sometimes being overwritten by older data in one direction or the other. Since you aren't downloading from financial institutions, that likely avoids some of those problems, so it might be a viable solution for you. Personally, I'm wary of using Quicken's cloud syncing, but in fairness to the product, I know there are many people who use it successfully. (The only other obstacle here might be if the \web browsers supported on Win 7 are so old that Quicken won't allow you to login with the old browser; I believe Quicken on the Web supports Chrome, Edge, Safari, and Firefox (but not Internet Explorer), but it might require a more recent version than you can run on Windows 7.)

Another possible option for you depends on what the MacBook would be doing when you want to access Quicken on the Win 7 machine. Unless someone else is using the MacBook at the same time, or you're running some process which requires the full resources of the computer (e.g video rendering), you could use simple screen sharing to open w widow on the Win 7 machine to show the screen of the MacBook, so you could run Quicken on the MacBook in the screen sharing window on the PC.