Hi Hassim,
In your case you have used ok_code type sy-ucomm..
Try the below code.
DATA ok_code type sy-ucomm.
case ok_code.
When 'FCODE'.
clear ok_code.
move ok_code to sy-ucomm.
"Process your data or select query.
Endcase.
So that it will clear ok_code and sy-ucomm and in next screen onward it will work fine.
Thanks,
Ramesh