compile error: Argument not optional

[QWERTY]

I have noticed one thing: in DisplayBill Function you have declared sftdrks twice. If you don't need it two times then delete it, otherwise in Private Sub cmdCompute take a look on your call of DisplayBill Function. It is missing one parameter (second sftdrks) at the end.
HTH

[r0ach]

You put the last Paramater (sftdrks As Single) in the call twice.
Remove the last paramater from Private Sub DisplayBill