you should use snap create-user ... to create a user … or better simply finish the first-boot wizard that comes up on first boot of the device …
while it is technically possible to add further users, Ubuntu core isnt necessarily designed as a multi-user system but for use in appliances (without user) or as headless embedded system. so adding more users (i.e. for development purpose or testing something) might require that you jump thrugh some hoops…