Your scoping is messed up. You define the functions as the following:
1. Enable
2. Disable
3. Event
4. UpdateSlot
5. Update
6. OnUpdate
While the functions tries to use them before they are defined:
Enable: Tries to use Event, which is defined as a local variable below and is nil for it.
Disable: Same as Enable.
Event: Tries to use Update and UpdateSlot, which both are defined below and are nil for it.
UpdateSlot: Tries to use OnUpdate, which is defined below and is nil for it.
Another note is that oUF will call the element update function on PLAYER_ENTERING_WORLD, so you don't need to register it.
That's the issues I can see at first
.