The message should be in your list of messages in the app. There would be a small symbol of a clock next to it to show it is scheduled. To edit the message or delete it, touch the message and hold it for options.
I just tested this on my S9. I opened a conversation, typed in the text, set the time to send two hours from now and then tap the send icon. It adds it to the bottom of conversation in a grey color. If I want to send it right away, edit or delete it I tap the small clock icon.
If you're not seeing this you might try clearing the cache files for the Samsung messages app.
Yes I have opened the conversation, closed it and closed the messages app multiple times and it's still there waiting to send. I also edited it, added more text, changed the scheduled time tap send. It's still there waiting. Another option that clears up some issues after a system update is to wipe the system cache partition. As a last resort you can always backup your messages to Samsung cloud and clear the cache and data. Set the messages app back up and test schedule messages.
I have found the solution....on single person text scheduling, you merely open that person's text conversation. While on a group of persons that you scheduled, you must retype in the same group of persons (in any order), ONLY THEN will that scheduled text appear. yay!