Make sure you have POKEBALLS before doing a raid!
I just did a Snorlax raid with 10 other people in my hometown. I was very surprised that so many people showed up.
Long story short, I was walking towards the raid catching everything in sight to get stardust to level up my Pokemon prior the raid. All of a sudden, I ran out of Pokeballs. I didn't think it was a big deal. I enter the raid and we beat the Snorlax. But then after the raid is over and it gives you the ability to catch the raid boss, I had a VERY HARD time GRABBING the pokeball because I had zero pokeballs and that circle, that you see in the below picture, was over the pokeball prevented me from grabbing the ball. I had to wait until it jumped up in the air a little bit and then grab it. It was an absolute nuisance! Anyways, I pulled through and managed to catch the Snorlax. Not the best moveset or IV's, but oh well. It's still a Snorlax. I'm very happy. :D
Got 6 rare candy & a charge TM. :D