The IRS says you did get the $600 stimulus payment so you cannot have it as a credit on the 2020 return form 1040 line 30.
If you disagree then have them put a trace on the missing payment ...
Go to this IRS website for information on initiating a trace if you never received a stimulus payment - https://www.irs.gov/newsroom/questions-and-answers-about-the-first-economic-impact-payment-topic-f-p...
