Charactermancer won't level up above level 5

I am a DM trying to make a antagonist for my party. I cannot level them up above level 5. I have tried to make a new game with nothing else other than the character and no luck. It just sits as if it is going to do it and nothing happens. I tried this last bit because I read that sometimes if you have alot of assets in a game it fritzes out. I also tried on Fire Fox to no avail.

Class, race, and sheet? Can you level them up to 5 and then save and then level them up further after that?

A weird thing happened I tried again on several character sheets and it let me do it. I cannot explain it I have been having this problem for months until now.

Did you use either the dnd starter set or essentials kit before. I think they only go to lvl 5.