Hello! I'm DMing for a Warlock PC and through the story they basically petitioned their patron for an additional spell - in my case he wanted to learn Remove Curse. It is on the warlock spell list and he could've chosen to take it when he leveled up but didn't at the time. He petitioned his patron and succeeded. How do I give him the spell? The "learn" button is greyed out and there is no option to add it in any way. Please help.

You would have to create a homebrew copy of their subclass, add it through that, and swap their character’s subclass from the original one to your homebrew version.

Thanks! I just ended up making a tattoo magic item and made it let him cast the spell. It's clunky and doesn't automatically use up his spell slot but oh well.