Did you run a backup before you logged out to change the encryption?
If you did, you won't be able to enable encryption - if it allowed you to do this, you'd have a mixture of unencrypted files and encrypted files stored on the online vault - some files would need decrypting, some wouldn't. The same would apply if you changed the key. Some files would need one key, some would need another. So things get extremely messy.
Only way to change encryption details when backups have already been running, is to login to https://wordpress.backup-technology.com/ and delete the backup data that is already there, (it will delay deletion for 24 hours to give chance to cancel it.) Once it is gone you can then start backups fresh with new encryption details.
If you don't want to wait, you could just register another account.
Let me know if you still have questions.