Again that is a general notice that lists several "possible" errors (among other not listed possibilities) and the only one that applies to most people is the last bullet point for what they consider a "math" error which in the TT program is a data entry error on the user's behalf.
If the IRS disagrees with the amount of the third stimulus payment you listed on the return and you believe they are wrong in that you did not get the payment in advance then you need to initiate a trace on the missing payment ...
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...
If you want call the number on the notice to ask for more specific information.
