Can't download my 2019 tax return from TT. Receive a message that it is located in a different acct. No idea what they're talking about. How do I do this?

You may have multiple TT Online accounts.  See:

How many Intuit Accounts do I have with TurboTax?

But if you used TT desktop (CD/download) to do your 2019 return, look on your computer hard drive for a .tax file.