I'm here to help you log in to your Desktop App for QuickBooks Online on Mac. Before that, I'd like to know if you've encountered any error messages when trying to login into QuickBooks App. If yes, you can share it in this thread so that I can provide a specific resolution.
You can try resetting the app data if there's no error to refresh your account and clear its cache and cookies. Here's how:
In the app, click Help in the menu bar.
Click "Reset App Data.
Select "Reset," then log in.
If the issue persists, I'd suggest uninstalling and reinstalling the app to refresh the system. You may also check out this link attached for more troubleshooting actions you can perform when you've encountered issues while using the app: QuickBooks App for Windows and Mac: General support.
I got your back if you have any other questions. Feel free to let me know in the comment section.