Scheduled posts need a time from now onwards. Times are in your device's timezone, shown under the date and time.
Other scheduling messages
"Posts can be scheduled up to a year ahead." Pick an earlier date.
"Only scheduled posts can be changed." This post has already been posted or cancelled.
"This post is already going out." It is due now; share it from the post.
"This post has already gone out." It cannot be cancelled.
"This post is already marked as posted."
What to do
Pick a later time, or choose Post now to share straight away.