It looks like the fieldcat definition of Requisitioner of PO is not correct. When you change the layout, the report automatically regenerate the ALV with all fields selected. The error shown by the compiler says that was not possible to assign the field of your internal table with the fieldname passed by fieldcat table.
E.g.:
CLEAR t_fieldcat.
t_fieldcat-fieldname = 'AFNAM' . << " check in your code
" this point, if the
" name definition is
" the same in your
" output table
t_fieldcat-tabname = 'OUTPUT_TABLE .
t_fieldcat-reptext_ddic = 'your text here' .
t_fieldcat-inttype = 'C' .
t_fieldcat-outputlen = 12 .
APPEND t_fieldcat.