Create User REST API getting - 403 Forbidden You do not have access to this page

Hello we have built and integration with Zendesk, where before creating a ticket we check if the customer/user exist if not we create it then we proceed with the ticket as well.
The endpoint is -> https://developer.zendesk.com/api-reference/ticketing/users/users/#create-user
All other calls as /api/v2/users/me.json or Ticket Creating/Listing work fine.

Our App ID is 279252

We have hundreds of merchants using the integration but only one is getting the following response. 403 - Forbidden

{
    "error": {
        "title": "Forbidden",
        "message": "You do not have access to this page. Please contact the account owner of this help desk for further help."
    }
}

For Authentication through the Zendesk API - OAuth Clients.
Let me know if you need more info about the case.