I tried that too and then got the message "The submission is a duplicate of one that was previously received.  Please contact customer support."

I guess I will have to wait to call tomorrow.

The initial reject said "If form 1040-X is present in the return, then ('AmendedReturnInd' or 'SupersededReturnInd") in the return must be checked.  

I don't know why I got that message but did "uncheck" file federal extension box.

I had an amended 2020 at the same time - it went through with no problems.

I had to file both because of amended K-1's due to ERC.

Thanks