I didn't get the 2021 Economic Impact Payment the IRS sais I got. What do I do?

When the program asks you how much of the third stimulus payment you got in advance simply enter the amount you actually got no matter how much the program says you are eligible for.  

If the IRS  sent you a payment you didn’t receive you’ll need to have them put a trace on it to find out where it went. Claiming it on your return when it’s already been received (according to IRS) with cause a delay in processing since the IRS records indicate you got a payment. 
contact IRS.gov and search payment trace.

You will need to contact the IRS and have a payment trace made on the stimulus credit the IRS thinks you received. Filing it on your tax return will result in a manual review since they think they sent the payment. Contact irs.gov and search payment trace and follow instructions to have payment traced. Look at FAQ’s of stimulus payments for more information on how to trace payment not received.