Where did you upload the secret_key.txt file to? You need to replace your_backup_secret_key_file.txt in that command with the actual location of the file on the server.
You have to copy over the secret_key.txt file from the old server and place it somewhere outside of the /home/user-data/ directory on the new server like you did with your backups. You are trying to use the secret_key.txt file from the new server. That is not going to work.
And do not run these commands from within the /home/user-data directory.
That is not what your message says. Actually, if you copied the old secret_key.txt file over to the new MiaB, you have overwritten the key, and potentially have destroyed the ability to create backups for yourself.