builder.Services.AddAuthorization(options =>
                {
                    options.DefaultPolicy = new AuthorizationPolicyBuilder(OpenIddictValidationAspNetCoreDefaults.AuthenticationScheme, CookieAuthenticationDefaults.AuthenticationScheme)
                        .RequireAuthenticatedUser()
                        .AddRequirements(new PermissionRequirement("default test!!"))
                        .Build()
                        ;

                    options.AddPolicy("MainPolicy", policy =>
                    {
                        policy.AuthenticationSchemes.Add(OpenIddictValidationAspNetCoreDefaults.AuthenticationScheme);
                        policy.AuthenticationSchemes.Add(CookieAuthenticationDefaults.AuthenticationScheme);
                        policy.RequireAuthenticatedUser();
                        policy.Requirements.Add(new PermissionRequirement("MainPolicy test!!"));
                    });
                    //options.AddPolicy("MainPolicy2", policy =>
                    //{
                    //    policy.RequireAuthenticatedUser();
                    //    policy.Requirements.Add(new PermissionRequirement("MainPolicy2 test!!"));
                    //});
                })
   ;

// Register the OpenIddict validation components.
        builder.Services.AddOpenIddict()
            .AddValidation(options =>
            {
                // Note: the validation handler uses OpenID Connect discovery
                // to retrieve the address of the introspection endpoint.
                options.SetIssuer(authUrl);
                options.AddAudiences("SingleClientAPI");

                // Configure the validation handler to use introspection and register the client
                // credentials used when communicating with the remote introspection endpoint.
                options.UseIntrospection()
                       .SetClientId("SingleClientAPI")
                       .SetClientSecret("430695D4-4F20-4F2E-A478-BB5DF6A7C543");

                // Register the System.Net.Http integration.
                options.UseSystemNetHttp();

                // Register the ASP.NET Core host.
                options.UseAspNetCore();
            });

        builder.Services.AddAuthentication(OpenIddictValidationAspNetCoreDefaults.AuthenticationScheme);
        builder.Services.AddAuthorization();

and "SingleClientAPI" has a controller with action "GetWeatherForecast" and authorization policy "MainPolicy" (supposed to be used from "AuthAPI"):
        [HttpGet(Name = "GetWeatherForecast")]
        [Authorize(Policy = "MainPolicy", AuthenticationSchemes = OpenIddictValidationAspNetCoreDefaults.AuthenticationScheme)]
        public IEnumerable<WeatherForecast> Get()
        {
            return Enumerable.Range(1, 5).Select(index => new WeatherForecast
            {
                Date = DateOnly.FromDateTime(DateTime.Now.AddDays(index)),
                TemperatureC = Random.Shared.Next(-20, 55),
                Summary = Summaries[Random.Shared.Next(Summaries.Length)]
            })
            .ToArray();
        }

System.InvalidOperationException: The AuthorizationPolicy named: 'MainPolicy' was not found.
   at Microsoft.AspNetCore.Authorization.AuthorizationPolicy.CombineAsync(IAuthorizationPolicyProvider policyProvider, IEnumerable`1 authorizeData, IEnumerable`1 policies)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext)
   at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)