Treasury Bills

Quicken certainly can track TBills and doing so is pretty much exactly the same process you'd use for any other security. 

You can create the security using Tools > Security List > Add Security.  Enter the name you want to use, (e.g., "US Treasury 0.125% 07/31/2023"), and tell Quicken you'll enter the information for the security, (which Quicken won't find), manually buy clicking on the "Click here" link down at the bottom of the "Add Security to Quicken" window.

There's no Ticker symbol for the bond so you can leave that box blank, then select "Bond" from the drop-down list in the "Security type" box.

In the next window select Treasury Bill from that page's Security type drop down list and enter the bond's maturity date.

At that point you can manually enter your buy of the bond using the Bonds Bought action.  Enter the number of bonds and the price.  The price entry is based on "100's" meaning that if you bought the bond at par - no discount or premium - you'd enter 100 as the price, a discount would be entered as something less than 100 (e.g., 99.873) and a premium would be entered as a number more than 100. 

If you're buying the bond directly from the US Treasury you'd create a "manual" (non-connected) brokerage Account if Quicken to track your activity.