We recently rolled out the Single Sign On for ZwiftPower so it’s no longer necessary to go through those extra steps of adding the code to your surname for account validation since the login process itself is now uses your Zwift credentials so you should be able to complete that process.
https://forums.zwift.com/t/single-sign-on-sso-is-coming-to-zwiftpower-january-24/572069
We’re currently working on the issue with the Submission failed error, but it should no longer impact your ability to register for ZwiftPower.