我會回來的 in English is "I will be back" or "I'll be back" but the phrase "I'll back" does not make sense.

They don't mean the same thing. "I'll back" is usually part of longer sentence/phrase, f. ex., "I'll back the proper (political) candidate", meaning "I'll support them". "I'll be back" simply means "I will return".

"我会回来的"是“I'll be back"或者"I'll return". "I'll return"更正式。 "I'll back"不对，因为'back‘不是动词（真的，'back'可能是动词，但是它的意思是‘支持’）。