In the program, it asked how much you received for each stimulus. Any amount that you did not receive, was added to your refund. Are you saying you received a stimulus check and then answered that you did not receive the second stimulus check? Your tax return would be wrong. The IRS may catch it in cross checking. If not, you will need to file an amended return and repay the money.

Thank you for reply 

If I received the 2nd stimulus in 2021, why I should file it in 2020?

The IRS began distributing them in Dec 2020 and Congress has chosen to have it count on the 2020 tax return. Why Congress does anything that they do, I don't know. So, you got it in the beginning of 2021 for a payment begun in 2020.

Thanks for the reason to include the 2nd check in the 2020 return.

@ TT this answer should be added to the help section.