Check your spellbook and see if your Sim has any spells left to learn from the sage he's trying to learn from. In my experience, this is a default message for "I don't have anything I can teach you right now." You'll get this message if your Sim has already learned all the spells from that sage's category.
It might have been better for the developers to add another message that specifically says you've already learned everything but I guess they didn't anticipate the need. They probably thought people would realize that they couldn't learn more spells if they already knew them all.
I tried to make it clear that I had a few spells left to learn. I checked the book frequently as I wanted my sim to learn the all the spells. She had 2 spells that she needed to learn. I should've taken a picture of the spellbook and what spells she needed to learn but didn't think of it. It doesn't matter now as I had my sim practice with her magic until she learned the 2 remaining spells. My sim now knows all of the spells. It's still a bug that should be looked into.
**Sorry for the late reply and I posted a reply using my other email account but had deleted it so they would be no confusion.
The ones I found are so old that I think they are closed or been ignored. Most of the "answers" are blaming mods. However since I never used a mod in my life it's clearly the game. **Edit to add** I only found this bug on random forums I don't think EA ever acknowledged this bug.
Until they fix the problem (if they ever do) your best bet is to just have your sim practice magic to learn spells. It took about 2 sims days for mine to learn the 2 remaining ones.
I am having this same issue. I am a Virtuoso spellcaster and I know all the spells except the last two Untamed spells. When I try to practice magic, it says I know everything I can for my current level. When I ask the Untamed sage, they say to rank up before they can teach me more.
