Raichu is one of few Pokemon that learn no moves by level up past Level 1. Since you are playing Moon, I assume you are talking about Alolan Raichu, (but whether it's Alolan or Kantonian doesn't make a difference here.)
The only way to get a Raichu with Electro Ball is to get a Pikachu with Electro Ball. When Pikachu reaches Level 13, it will learn Electro Ball. Raichu cannot learn Electro Ball by itself.
If you have questions just ask them .
