error in BDC

hi,

im doing a BDC on tcode VA01

and im inputting PO number for each item,that data is found on the tab page Purchase order data and its name is VBKD-BSTKD, screen number is 4003

but when im execting the BDC, an error is being displayed saying that the field VBKD-BSTKD does not exist on screen SAPMV45A 4003, but when i used SHDB to record the transaction, this field was on that screen itself

does anybody know y this error occurs?