Hello and welcome 
to the community.
I think that if you need Admin users you need to hit up admin/register etc 
I can’t remember the exact endpoint but I think that there was some guide on discord or on the forums which listed it.
I think it’s admin/local/register to register admin accounts.
If not you can always just create your OWN endpoint, and register admin users that way.
api/users is used for API users not for admin credentials iirc.