Same problem here. I'm unable to open the return to do an amended return. I have used the same Mac laptop and TurboTax Premier CD since version 2015 without any problems. Previously, years on a Windows machine without a hitch. This year (possibly because the current tax software might not be compatible with the new Apple OS?), I had much trouble saving it and reopening it. Also, it retrieved last year's information in only a few places; most of it had to be entered manually.
Last month, after several hours on the phone with a very helpful Help Desk person, I was able to open it and work on my return. Fearing I would not be able to reopen it, I filed it electronically with just three rounds of checking and without "sleeping on it" as usual. Result: I had overlooked one IRA RMD (paid out without a mailed document before skipping it became a legal option). Now, trying to open the same return to amend it, it will not open. First it asks me to download TurboTax updates, then it stalls without opening. After restarting the computer, the same update request and the same stalling. I presume I will have to manually fill out a printed form and mail it in.
