Can you provide a bit more context about how you expecting your app to behave?
A first glance, it would seem that you are attempting to mix the device OAuth Grant flow with the entity management feature; which I would have to check and see that if that is possible.
OAuth endpoints
https://fusionauth.io/docs/v1/tech/oauth/endpoints/
Possibly related issue?
https://github.com/FusionAuth/fusionauth-issues/issues/1295
Thanks,
Josh