compile error: Argument not optional

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

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