"shouldn't the software fill it in or attach?"
That's your job.
In other words, you have to explain each line with Col B on 1040-X in Part III.
If there is a form or schedule showing a new $500 credit, you have to attach that, or,
as you have learned, IRS will send the amendment back to you.
@mmovses
IRS has invited you to try again with another 1040-X submission.
