Solved
I already filed my return with the desktop application, but need to update the filing for new 1099 that arrived late. Unfortunately this is a notoriously broken feature for Mac users and Intuit Support directed me to upload the return and make the modification. (They gave me a code to allow me to file the online version for free.) I'm logged into my account but don't see an option to bring the existing return into the system.
See this thread. Might have some work arounds.
https://ttlc.intuit.com/community/taxes/discussion/blank-screen/00/3224366
Or one thing you can do is to find a Windows computer to switch to. Then you can move your Mac return to Windows without starting over. Then you might be able to efile the amended return.
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.