Use 'sit out' to express a deliberate choice not to join an activity, often for personal reasons. He chose to sit out the debate competition due to a lack of preparation.
Pro Tip 2/3
Temporary Non-participation
'Sit out' often implies a temporary absence, with the intention of returning later. She will sit out this game but expects to play next week.
Pro Tip 3/3
Not Negative
Choosing to 'sit out' doesn't always have negative connotations; it can be a neutral or strategic decision. The coach agreed it was smart for him to sit out the friendly match.