I am amending my federal return. But when complete line 7 has a zero on the 1040X form but line 21 on the 1040 form is blank this is causing an error they should match?

How do I either add a zero to the 1040 line 21 or get rid of the zeros in line 7 on the 1040X because I have retried the entire amend process three times and have not gotten it to correct this problem.