I received a letter from IRS saying that I owe more than what my return shows. How can I find out what went wrong?

 

After you file

The letter should explain the discrepancy. 

After you file

How much more do you owe?  A common adjustment is if you claimed the Recovery rebate credit on 1040 line 30 when you already got the first 2 Stimulus payments or got the second one after you filed or reported the wrong amounts or entered them in the wrong box.  Do you have an amount on line 30?