Why can't I add the institution One Main Financial?

@AuthenticLinda If you are asking if you can add this as a manual account, the answer is yes. You'd enter your transactions manually, and Quicken would reflect your balance and your income and expenses would be categorized along with your other accounts. To do this, when you start to add a new account, click at the bottom of the window where is says "Offline Account". (It doesn't look like a button or link, but clicking in that area will move you forward.) You then specify the account type and starting balance and the account will show up.

If you are instead asking if there is a way for Quicken to add One Main Financial for downloading, the answer is no. The financial institution needs to sign on with Quicken (through their service provider, Intuit); if they do so, they will then show up in the directory of financial institutions. So you'd want to reach out to One Main to ask them to add support for Quicken and see what their response is.