Form 1040X 2020 filing error message

[rkaffen]

I attempted to file a form 1040X for 2020 due to the fact that a 1099 withholding was overlooked on the original filing. The problem was mostly caught by IRS that issued a refund check to the customer that exceeded the requested refund. However, there is still an additional amount owing to the customer. The tax on line 6c is the same amount as the original filing. The only change is the amount withheld on line 12c. This figure was brought down to line 17. Line 18 equalled the amount of refund as adjusted by IRS. Line 21 shows the amount still owing on the refund. When this form is efiled, I get a message that line 6c must equal the amount showing on lI

[Just-Lisa-Now-]

what did IRS adjust for? Was it the unemployment exclusion? 

Im not sure what "must equal the amount showing on lI" means