I fixed this before. I believe I had to go into database and delete all the past revisions for that page. Though I think in my case I didn’t delete the page. I changed the permalink and was trying to apply the former slug it to another page. It may be the same fix.
DELETE FROM wp_posts WHERE post_type ='revision' AND post_id = 1; whatever is the post_id.
Did you accidentally delete it from the Trash too already? If it is still in your trash you can recover it and start using the old page again.