Just remove the position 50 on both comments (else activation will fail)
SELECTION-SCREEN BEGIN OF LINE. PARAMETERS p_a RADIOBUTTON GROUP grp1 DEFAULT 'X' USER-COMMAND switch. SELECTION-SCREEN COMMENT (10) text-001 FOR FIELD p_a. SELECTION-SCREEN POSITION POS_LOW. PARAMETERS p_yes RADIOBUTTON GROUP grp2 DEFAULT 'X' USER-COMMAND switch. SELECTION-SCREEN COMMENT (10) text-002 FOR FIELD p_yes. SELECTION-SCREEN END OF LINE. SELECTION-SCREEN BEGIN OF LINE. PARAMETERS p_b RADIOBUTTON GROUP grp1. SELECTION-SCREEN COMMENT (10) text-003 FOR FIELD p_b. SELECTION-SCREEN POSITION POS_LOW. PARAMETERS p_no RADIOBUTTON GROUP grp2. SELECTION-SCREEN COMMENT (10) text-004 FOR FIELD p_no. SELECTION-SCREEN END OF LINE.
(An yes with those COMMENT FOR FIELD SCI will be less prompt to protest 
)
Regards,
Raymond