The message is likely coming from the Server. The "mobilepos" Application has not been configured in
Users > Applications.
Note that the Application Identifier is the client_Id that is used in the Mobile App, and that it is probably hard-coded as "mobilepos" so it must match exactly in the SambaPOS Application configuration.
The User that you use to Login from the Mobile POS must also be configured with a Password in SambaPOS. The PIN does not work when Authenticating the Application.
After you configure the Application (and a User with a Password), you should restart the SambaPOS MessagingServer Service using the Windows Services interface. This needs to be done when you change certain things in SambaPOS, including the registration of an Application.
You should also configure the Service to run with a User Account rather than a System Account.