Getting a runtime error when using a standard Form Interface

I am getting this Error analysis:

The following syntax error was found in the program /1BCDWB/SAPLSM00000212 :
"Field "VBRK-WAERK" is unknown. It is neither in one of the specified t"
"ables nor defined by a "DATA" statement. ."
" "
" "

The problem is, I am using a standard interface INVOICE_INTERFACE for a custom form. Why am I getting this error? Where in my form should I declare VBRK-WAERK for my program to work?