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.”