It looks like the federated Hybrid join flow is failing "azure registrationType: fallback_sync".
So, if you have Windows 10 clients which are 1803 and above, then when the federated flow fails, the client will automatically try the managed method. In this method, the computer object needs to be in sync scope.
Based on the error, it looks like the user certificate object populated on the computer object in AD is not yet synced to the cloud.
If you want the federated method to work, I would start with the ADFS claim rules and also check if you are getting this error across multiple devices?
For the fallback method to work, let the sync cycles run and then restart your device once. This should trigger another device registration request.
Update: I checked the device and the certificate is populated now. If you restart or logout/login again , Device registration should complete now.