Some time means a period of time.

For example:-

“It took Lynne some time to spot Hekner’s post.”

Sometime indicates a time in the future or the past which is not known or not stated.

For example:-

“They hoped to meet up sometime.”

Not to be confused with sometimes of course, which means on some occasions but not always or often.

For example:-

“Sometimes Lynne misses posts in the forum.”