Just to confirm, does your app have the necessary permissions/scopes to access the user data? Your app might lack the necessary access scope for the User resource, specifically the “read_users*”* or *“*write_users” scope.
Also the User API returns information about the staff members of a Shopify store, not the customers. It sounds like you’re trying to get information about a logged-in customer, not a staff member. In this case, you would need to use the Customer API, not the User API.