OAuth 2.0 keep getting Authorization has been denied for this request

Question

I am new to ASP.NET and OAuth and I have an ASP.NET project that uses OAuth 2.0, but every time I send a request to the API in Postman, I get back a message

Authorization has been denied for this request

even with the bearer token attached in the header. The token is generated in a separate project.

This is my Startup.cs:

using Microsoft.Owin;
using Microsoft.Owin.Security.OAuth;
using Owin;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Http;

[assembly: OwinStartup(typeof(IMOTBaseWebAPI.Startup))]

//Added by Danish 25/3/2024 

namespace IMOTBaseWebAPI
{
    public class Startup
    {
        public static OAuthBearerAuthenticationOptions OAuthBearerOptions { get; private set; }

        public void Configuration(IAppBuilder app)
        {
            // For more information on how to configure your application, visit http://go.microsoft.com/fwlink/?LinkID=316888
            ConfigurationOAuth(app);

            HttpConfiguration config = new HttpConfiguration();
            WebApiConfig.Register(config); //Already called in Global.asax Application_Start
            app.UseCors(Microsoft.Owin.Cors.CorsOptions.AllowAll);
            app.UseWebApi(config);
        }

        private void ConfigurationOAuth(IAppBuilder app)
        {
            OAuthBearerOptions = new OAuthBearerAuthenticationOptions();
            //Token Consumption
            app.UseOAuthBearerAuthentication(OAuthBearerOptions);
        }
    }
}

This is my global.asax.cs:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Routing;

namespace IMOTBaseWebAPI
{
    public class WebApiApplication : System.Web.HttpApplication
    {
        protected void Application_Start()
        {
            AreaRegistration.RegisterAllAreas();
            GlobalConfiguration.Configure(WebApiConfig.Register);
        }

        protected void Application_BeginRequest(object sender, EventArgs e)
        {
            var application = sender as HttpApplication;

            if (application != null && application.Context != null)
            {
                application.Context.Response.Headers.Set("Server", "");
            }
        }

        protected void Application_PreSendRequestHeaders ()
        {
            Response.Headers.Remove("Server");
            Response.Headers.Remove("X-AspNet-Version");
            Response.Headers.Remove("X-AspNetMvc-Version");
        }
    }
}

Is there anything I'm doing wrong? Is the Startup.cs is executed automatically or do I need to call it from somewhere?