After you file

The missing amount is $600 ?   The amount of the second stimulus check ?   Look on your return ... form 1040 line 30 ... is the missing amount there ?   If so you said you did not get the second stimulus payment and the IRS says you did ... this was not a TT error.   So look at your account on the IRS site ... do they indicate the payment was sent ?   And did you get it but forgot ?   Or got it after you filed ? 

https://www.irs.gov/payments/your-online-account