I as well was very dissatisfied when I discovered that this option was not available it was simple as clicking the option button on the top the free darts and scheduling a text when I was working late at night but thanks to this community I have found this little bit difficult to find so I have added a photo to help everyone else as I was convinced the option was not present I hope this helps anybody else going through the same distress as me.
1. Tap + symbol bottom left of the screen
2. Tap 3 dots in that row Not the 3dots on top corner.
3. For me it's the second option
Running a Note 9 with Oreo through AT&T and I got it on mine just fine. Type your message, Hit the + sign next to the message field, click the 3 dots on the left, and select Schedule Message. I use it all the time for reminders.