Are you sure it’s being triggered?
You can place f.ex. a PrintString to show a debug text to make sure if it all happens and instead showing ‘Hello’ connect the ToText Return Value to it to show what it returns
Also when you Level Up - do you want to start from 0 Exp or rather keep the value that exceeds Max Exp Needed to Level Up and add to the Next Level?
F.ex. - You need 100 xp to level up, you have 90 and you just got 20 more so total you have 110 xp - it’s 10 more than necessary - so next Level should already have this value
The calculation is simple for that one too 
Actual Exp - Exp Needed to Level Up = Exp over
110 - 100 = 10
And this is the value you should add to the exp after leveling up 
Let me know if the OnCharacterLevelUp gets triggered, also - is that Multiplayer?