The stimulus check is an advance on a credit you can receive on your 2020 tax return.  If something went wrong or you did not get the stimulus check this year, you can get it when you file your 2020 return in early 2021—if you are eligible.It will end up on line 30 of your 2020 Form 1040.

https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-j-reconciling-on-your-...

https://www.irs.gov/coronavirus/coronavirus-tax-relief-and-economic-impact-payments-for-individuals-...

https://ttlc.intuit.com/community/tax-topics/help/how-will-the-stimulus-package-impact-me/00/1393859

At this point, we do not know yet what the new stimulus bill will mean in terms of changes to the 2020 Form 1040, software programming, software updates, etc., etc.  Do NOT be in a hurry to file your 2020 tax return until more is known. Give some extra time to the IRS and the software programmers so that the software can handle the new 2nd stimulus, so that you can get it all by filing ONE time.   It is never a good idea to file early, and it would be especially foolish to file an early 2020 return.

When you file your 2020 return there will be questions about whether you were entitled to stimulus money and did not receive it.   The amount that you did not receive will appear on Line 30 of the 1040 form.  That money is a refundable credit.  It will be used first to pay any taxes due and then the rest will be refunded to you.

interesting, what happened with the 1st stimulus - did you get the right amount?  I am just curious, but the advise posted by others is going to make it easy to correct this!