dUMP:A new value is to be assigned to the field "<L_BOX>"

DUMP:

A new value is to be assigned to the field "<L_BOX>", although this field is entirely or partly protected against changes.

key:

probably you have specified the fields box_tabname and box_fieldname of structure 

FORM pf_alv_list_layout USING ps_layout TYPE slis_layout_alv .
  CLEAR :  ps_layout.

  ps_layout-colwidth_optimize    = 'X'.
  ps_layout-detail_initial_lines = 'X'.
  ps_layout-zebra                = 'X'.
  ps_layout-box_fieldname        = 'SEL'."这句的问题

ENDFORM.