TurboTax home and business ... does the Federal Tax Due amount include 2021 stimulus payment if not received when filed???

If you are referring to the 2021 third stimulus payment, no that is not included in the 2020 federal tax return.

If you are referring to the Recovery Rebate Credit and you were eligible for the credit based on your 2020 tax return, filing status and 2020 AGI for stimulus payments 1 and/or 2 that you did NOT receive, those credits would have reduced the 2020 federal taxes owed on the 2020 federal tax return.

The new 3rd Stimulus payment?  No.  Your 2020 return only reconciles the 1st and 2nd rounds, but the second round could have come in January or February 2021.   If you didn't get the right amount for the first 2 payments or qualify for more it should be on 1040 line 30.  Switch to Forms Mode and see.  Be sure you don't have an amount on line 30 you already got.