Function Code error while enabling/disabling Check box

Hi,

I get an error because of a function code added to a check box in a screen. The requirement is to enable a field if the check box is ticked and disable it while its not. I created a function code for this.

When i execute the transaction and tick the check box, i get this error

Requested Function ABCD is not available here.

Note: I have not written any code in PAI or PBO yet and this is a customer exit enhancement on a standard transaction. This check box is mapped to dictionary structure.

Could anybody suggest on this?