If the IRS doesn't allow electronic amendments to 2021 1040 forms, turbotax shouldn't give customers the option to submit them.
I got the same error and I think the trouble is the result of a bug in the IRS intake code that checks to see if all the entries for the dependent you're trying to remove match. In my case the value for the variable 'EligibleForODCInd' doesn't match because I'm not eligible for the "Credit for other dependents" if I'm no longer claiming the kid as a dependent. The amended return gets rejected if they don't match, but they shouldn't match, and you cannot force them to match.
After spending hours with eight(!) different turbotax customer service people, I have concluded that the only option to deal with this is to mail in a hard copy of the 1040-X.
