What is the ruling on someone who is not proficient in prayer leading the prayer?

If by "unable to pray" is meant the imamate of a person who is incapable of performing some of the pillars of prayer, such as bowing (rukūʿ) and prostration (sujūd), then his imamate is not valid for one who is not similarly incapacitated, according to many scholars. As for the ruling on the imamate of one who is ignorant of the rulings of prayer, that has been detailed previously.