For some reason, I can't level up when I rest or sleep. My level up bar is at 13/10, but the level up window doesn't appear! Has anyone else had this problem, and how did they fix it? Thanks.

I'd like to help, but since I haven't played in such a long time, my only suggestion is to reinstall the game. or reapply the last patch. What mods are you running?

Better Bodies, More Summoning, Magicka Spell, Magicka Amulet, Tribunal and Bloodmoon, and the official plugins.

The only thing I can think of is disabling auto-save on rest. The option is somewhere in the main menu.

The auto-save feature can be a bit "quirky" to put it politely.