@mikey123486
That letter has a laundry list of possible issues but the only one that applies is the Stimulus credit for $1400 on line 30 of the form 1040. You said she got it in advance then she cannot also get the credit on the return ... that is what the IRS corrected.
The first 2 stimulus payments ( 1200 sent out in 2020 and the 600 one sent out in early 2021) were reconciled on the 2020 return ... so if she did not get the first 1200 payment look at the 2020 return line 30 ... did she get the credit on that tax return ? If not the 2020 return should be amended.
