PowerBIClient Forbidden issue

You are mixing the directions for user versus app authentication.  If you plan on the users not having accounts you don't need to pass in or use a clientID and SecretKey.  Here is a code example.  You will just need a username, password and clientID to login.  If you want to opena report and/or dashboard you also need a workspace guid.  Let me know if you have any questions.

     var credential = new UserPasswordCredential(Username, Password);

            // Authenticate using created credentials
            var authenticationContext = new AuthenticationContext(AuthorityUrl);
            var authenticationResult = await authenticationContext.AcquireTokenAsync(ResourceUrl, ClientId, credential);

            if (authenticationResult == null)
            {
                return View(new EmbedConfig()
                {
                    ErrorMessage = "Authentication Failed."
                });
            }

            var tokenCredentials = new TokenCredentials(authenticationResult.AccessToken, "Bearer");

            // Create a Power BI Client object. It will be used to call Power BI APIs.
            using (var client = new PowerBIClient(new Uri(ApiUrl), tokenCredentials))
            {
                // Get a list of reports.
                var reports = await client.Reports.GetReportsInGroupAsync(GroupId);

                // Get the first report in the group.
                var report = reports.Value.FirstOrDefault();