When I say, " I'll see you sometime later" or " I'll see you some time later" ?

As one word, it means "at an unspecified time". As two words it needs "at", but it has the same meaning. "Sometimes" is always one word, and it means 'occasionally'.
To me, both are acceptable, but they have a very slightly different emphasis.

"sometime later" just means at some unspecified time later.

"some time later" means a measured amount of time later, often a significant or considerable amount of time later (compare "This will take me some time").

Because of the nature of this sentence, "sometime" seems more likely.