Invisibility Spell Skill Requirement Problem

The exact same thing happened to me too, I believe it has something to do with whether you are the appropriate level in respect to being a 'Journeyman', or 'Apprentice' or 'Expert' etc..

Although you may be above level 50 or a little above (like we both were) you are still not able to use the spells on account of not having the right class level yet.
Saying you need to be level 50 must then be a bit of dodgy-ness on the game's part. If you don't want to spend a whole lotta time casting illusion spells on unwitting subjects, the appropriate illusion trainer for Apprentice level to Journeyman is Carahil, in the Anvil Mage's Guild.

It happened to me to but I found out that if you go to your characters added spell page example: (+15%resistant to fire.) If it says (+7or8 more illusion) you have to subtract that from the level it says you are. Then get up like two or a lot more lv. When you get to level 50 it will show your change of class say a discreption and then you can use level 50 spells. If you don't want to level up from a trainer the best way to level up illusion that works for me is using a charm spell on someone to make them like you. Just keep using over and over untill you run out of mana, then wait a hour by pressing select and one hour. You will level up pretty fast. I did it three time on level 59 and I leveled up to sixty.