Just a warning to Ubuntu Core device builders out there of an incompatibility between Snapcraft 7 credentials generated with snapcraft export-login and using them with snap download and ubuntu-image (at least with snapd version 2.56.2).
There are several bugs already captured for this incompatibility
and forum post for workaround: Not able to download snap at required revision to build image
In the meantime, you can generate credentials by switching back to snapcraft 6, e.g:
snap switch --channel 6.x/stable snapcraft; snap refresh snapcraft, generate credentials and then use those with ubuntu-image and/or snap download.