watch and earn quest will not let me complete it

You must watch at least 15 minutes of the match, as far as I know. It worked for me the very first 15 minutes I tried it... I simply waited for a live match, clicked from the client, watched for less than half hour, went back to client and mission was completed. Just wanna add... the "Professional" league match I watched was silver elo worthy, kinda mad me mad watching it because me being a hard stuck silver player with 4k hours is literally laughing at the goofball shit happening in the match...

If it doesn't work then try to opt out and opt in again. It worked for me at least.

I believe you have to go to the watch page and go to rewards. Once you go there, the quest will complete and you'll get your reward. At least, that's how it happened to me.

same for me watched a whole match it was fuking boring btw and never got anything btw was the last time i ever watch something for a reward fucking waste of time

doesnt work for me till this day people!! Riot games please fix my account i've lost worlds quests like that already , dont make me lose another chain of quests please!