a full serial vault is needed to access a brand stores as others have said, also over time other features might also rely on device identity (for example some variations of optional snap features, where the store keeps tab of what is enabled (think IAPs)).
So far for operating just with the default store the error in the log is harmless (but will appear a few times per device per reboot and at least once a day).
We have plans (mostly related to development) to make it easier to get “weak” device identities, is just taking a while wrt other priorities: Opt-in accepting/distribution of 'generic'-signed serials Those kind of identities will stop the errors, will probably not be usable usually with brand stores.