PLEASE HELP! I see where there wasn't a single good solution to Brian's problems. I am having the EXACT same issue that he is. The problem is NOT anything having to do with his process or his data. This process worked fine last year for me. After your update it does NOT. What he was saying is that he simply took last years file and tried to export it and re-import it without him touching the file at all. That did not work and he got the same error that I am getting (below).

[ERROR] It's a list or transaction that is not supported by the IIF import process.

I'm editing my initial post without the anger (sorry about that). The following link had some helpful tips and I think the process is working now.

https://bigredconsulting.com/quickbooks-2019-iif-import/