Unable to add a screen field to FIELD statement

In PAI, I have the following code:

LOOP AT it_ pstdcs.

     CHAIN.

        FIELD:   /xyz/st_zxppstdocsn-ref_docnum .

        MODULE check_values .

     ENDCHAIN.

it_ pstdcs is an internal table attached to a table control. So in PBO, the corresponding LOOP AT statement is given for table control.

As per the above code in PAI, the screen field /xyz/st_zxppstdocsn-ref_docnum is prepared for input again if an error message is passed.

/xyz/st_zxppstdocsn-ref_docnum is a column within the table control.

So far so good. Now I want to prepare another screen field also for input, called /xyz/st_zxpepcmastn-issueamount This screen field does not belong to the table control.

I added this field name to the FIELD statement:

LOOP AT it_ pstdcs.

     CHAIN.

          FIELD:   /xyz/st_zxppstdocsn-ref_docnum, /xyz/st_zxpepcmastn-issueamount .

          MODULE check_values .

     ENDCHAIN.

This does not work, gives an error

What is wrong with this and how can it be worked out?

Thanks,

Kris