If your e-file is still pending, you won't be able to make changes until the IRS either accepts or rejects it. If your return is rejected for some reason, you will be able to add it at that point and refile. If your return is accepted, you will amend your return.
Please follow the directions on the How do I amend my 2020 return? if your return is accepted.
The AZ Form 301 credit should have been in the final part of the Arizona interview. I am not certain when you e-filed, but it is possible the form was not available when you submitted your return. I have confirmed that Form 301 is available now.
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