Read that notice again carefully ... note the put a list of "possible" errors but most likely the only one that applies to you is the last one for the incorrect stimulus credit or what they call a "math" error. That is the most common error for that notice so did you enter the incorrect amount of the third stimulus payment in the program when asked? Is the missing amount a factor of 1400?
Here is a link to the TurboTax Phone Number.
