which expression(s) is(are) right?

"a conflict of schedule"
"a conflict of schedules"
"conflicts of schedule"
"conflicts of schedules"
"a conflict of scheduling" 

Honestly, I'd say "Sorry, we seem to have a schedule conflict. Would early the following Monday work instead?"

I've certainly heard "scheduling conflict" as well as schedule conflict.

Your versions with the "... of schedule/s/ing" don't sound natural to me.
Students: We have free audio pronunciation exercises.
sorry if i wasn't explaining the question clearly - so what if the sentence is
Due to " xxx " , we need to make a rain check.
In this scenario, two people were trying to make do with their scheduleS to
do something together. The issue for me is
(a) there are TWO schedules, person A's and person B's
(b) there is a problem of "scheduling" 
(c) conflict is a countable noun
so I wonder if I should say
Due to
(a) a conflict of schedule
(b) a conflict of schedules
(c) conflicts of schedule
(d) conflicts of schedules
(e) a conflict of scheduling,  we need to make a rain check.
Or are you suggesting that we should say, 
"Due to a schedule conflict, we need to make a rain check" 
Thank you
Yes, "Due to a schedule/scheduling conflict, we need to take a rain check."