I had the same problem here. I ran the command specified (the one that appears on you screenshot) to generate the plain secret, but it did’t work. And even worse, when I ran occ upgrade command, it informed that upgrade was successful, but every time I tried to access via browser, I receive an “Update in process.” message.
Then I found this solution: https://help.nextcloud.com/t/update-to-13-fails-core-shipped-json-is-not-available/27294/4. It’s for an old Nextcloud version, but I just did it and worked perfectly. Basically it indicates to manually copy the download files from upgrade to Nextcloud directory (and set permissions properly).
But there was a trick. At least in my setup, the upgrade files were on my Nextcloud data directory and not on my Nextcloud install directory. Hope this help!