Prueba asi
SELECTION-SCREEN BEGIN OF BLOCK radio WITH FRAME.

PARAMETERS:
pa_dias type c default 'A',
semanal RADIOBUTTON GROUP mark DEFAULT 'X' USER-COMMAND ent ,
mensual RADIOBUTTON GROUP mark.
SELECTION-SCREEN END OF BLOCK radio.

AT SELECTION-SCREEN .
IF semanal = 'X'.
pa_dias = 'A'.
ELSE.
pa_dias = 'B'.
ENDIF.

START-OF-SELECTION.