I’ve never been able to get this working… I had another go today and it failed.
The option is there for me, I click to schedule something and every single time I get the same error saying Scheduled Publishing failed. The items are in draft initially, and then move to a Staged for Publish state which indeed does publish them at the next publishing action. I just can’t see what I could be doing wrong.
I have reviewed Sched. Pub. and I cannot find why you are having such issues. Can you create a video of your process and send to me. Also do me a favor and send me an updated share link for the project?