Can't level up after level-50 in borderlands 2 in True vault hunter mode
Solution 1:
Leveling up past 50 requires the Ultimate Vault Hunter Upgrade Pack, which is paid DLC not included in the base game. It's part of the first Season Pass, and part of the GOTY edition.
In addition to raising the level cap, it adds a 3rd playthrough option (Ultimate Vault Hunter Mode) but the two are independent.
You won't be able to gain any more levels until you've ponied up the cash.