There seems to be an issue when it comes to uploading older worlds to realms on the PS5 version of bedrock. I originally tried with my long-term survival world which is around 200mb. The Play on Realms button was grayed out but I was able to start the world upload in my actual realm worlds screen. The upload went to 100 percent. Then the screen stayed there for a minute and next thing you know there was an error message saying it failed to upload. I have since tried this several more times with other older worlds in my game with the exact same outcome. One world was only 3mb and still would not upload. I tried changing wifi, removing packs from the world, restarting my console and the game. I even tried uploading a slightly older copy of the exact same survival world from my pc to realms and it was faster and managed to work so I know the world is not corrupted. The only way I managed to get any world to upload on PlayStation was to make a brand-new world then immediately upload it afterwards. All of my older worlds that have been on PS5 for a while have the greyed out play on realms button and fail actually uploading to the realm. These worlds have also never had experimental features on or any kind of behavior/texture packs. From what I have read on reddit this seems to have been a problem several people have had for a while and has still yet to be fixed. It also seems to only be an issue on PlayStation as far as I can tell. Nobody has found any kind of work around for this as far as I can tell.
Environment
PlayStation 5
Duplicate of REALMS-474