It sounds like your a Realm file still has Users in it - or - the code still has a Users object defined. Try opening the Realm file with the Realm Studio app and deleting any existing User objects.
You can also try adding your User object back into the code as an object but include a _id primary key. Since it’s additive it should just add it. Once done, you will then be able to remove the object.
Lastly, you could just delete the local Realm file totally and let it rebuild the next time it sync’s.