That error message happens when one or more amounts are entered in Uneven Maryland Withholding but there is no MD withholding on Forms 502 line 40 or 505 line 45.
That's a section of the TT/MD Underpayment part of the program and you should go back there and remove your entry(ies) (there are 4 fields) for uneven MD withholding since there is no MD withholding.
