Basically, the game forces you to choose two spells when leveling up; until you do, you can't finish levelling. If you have insufficient spells available to learn (by way of scribing all spells that could be available to learn on level up), your Wizard gets softlocked at their current level. Can only happen on even levels, and only if you've been really meticulous about collecting spell scrolls. Just be sure to leave at least two spells unlearned going into even level ups, and you won't have any issues.