_G["ArenaEnemyFrame"..i]; return nil if "Blizzard_ArenaUI" isn't loaded.

LoadAddOn("Blizzard_ArenaUI") on initialize fix it.

What is the error you're getting?

Blizzard_ArenaUI is a dependency in the .toc, so it should always be loaded.

My .toc wasn't updated for some reasons, the dependency fixed it.