Generation canceled: No field with this name - LSMW IDOC Method

Dear Friends

I am trying to upload flat file data of purchase order through IDOC method LSMW then I am getting the error message "

Generation canceled: No field with this name ".

I have two flat files one for header and another one for footer.I followed the guide, even though the above error accours

Please guide me.

Thanks

Rajakumar.K