Can I file an amended 2020 federal return, when my tax liability does not go to zero? it shows 313 refund, then changes to 600 owed. this is not correct?

I am willing to bey that since it was a 2020 tax return, the $600 amount has to do with a stimulus payment. The program might automatically calculate a $600 payment made from the original return and when you begin the 1040X, it is assuming that $600 was not warranted.

We would need to know what was the 2020 tax result, reason for amending, whether the stimulus was part of your refund or reduction of liability, in order to get a more concrete answer. However, the $600 swing most likely has to do with the stimulus payment.