UC20 cannot enter '@' sign to log in

There is work going on to support easier onboarding mechanisms in the future, but i don’t know where on the roadmap this lives currently, might be a UC22 feature …

meanwhile you could use a system-user assertion or build your image with a config snap package or cloud-init.conf file for automated image onboading …

https://ubuntu.com/core/docs/reference/assertions/system-user

On a side note, @ is AltGr+Q IIRC