Hi Asha,
Can you check the configuration of output type via VF02.
1.Put the billing document and click on Goto-->Billing item over analysis.
2.Now click Goto --> Header-->Output.
3.Make sure the output type assigned there is of Green status and medium chosen there is Print output.
4.If Red means click on the error Further Data and check for errors.
Make sure you are chossing the right output type in VF03.
Also make sure you have captured the right generated function module in-case hard coded in the program.
Hope it helps.
Regards,
Kannan