Operation returned an invalid status code 'Forbidden'/'NotFound'

amatthews-yakchat -It is difficult to find error. Instead of , you can refer the below sample with Azure AD V2, Also compare this code with your own.

Sample Link-Microsoft-Teams-Samples/samples/app-complete-auth/csharp/AppCompleteAuth at main · OfficeDev/Microsoft-Teams-Samples (github.com)

In the below repository, you can call the dialoge  BotSsoAuthDialog in PromptStepAsync method. It will return the token and user details.

Dialogs link- Microsoft-Teams-Samples/BotSsoAuthDialog.cs at main · OfficeDev/Microsoft-Teams-Samples (github.com)

Repository Link-Microsoft-Teams-Samples/MainDialog.cs at fb5beb01271099430655ea0e56e8b6230c0e424e · OfficeDev/Microsoft-Teams-Samples (github.com)

 private async Task<DialogTurnResult> PromptStepAsync(WaterfallStepContext stepContext, CancellationToken cancellationToken)
        {
            if(stepContext.Context.Activity.Text == null || stepContext.Context.Activity.Text.ToLower().Trim() == "usingcredentials")
            {
                return await stepContext.BeginDialogAsync(nameof(UsernamePasswordAuthDialog));
            }
            if (stepContext.Context.Activity.Text.ToLower().Trim() == "sso" || stepContext.Context.Activity.Text.ToLower().Trim() == "logoutsso")
            {
                return await stepContext.BeginDialogAsync(nameof(BotSsoAuthDialog));
            }
            else if (stepContext.Context.Activity.Text.ToLower().Trim() == "facebooklogin" || stepContext.Context.Activity.Text.ToLower().Trim() == "logoutfacebook")
            {
                return await stepContext.BeginDialogAsync(nameof(FacebookAuthDialog));
            }
            else {
                await stepContext.Context.SendActivityAsync(MessageFactory.Attachment(GetLoginOptionCard()), cancellationToken);

                return await stepContext.EndDialogAsync();
            }
        }

According to the above error log, this error response occurs when it returns the invalid status code.
ErrorResponseException-Thrown when the operation returned an invalid status code.
Ref Doc-Conversations.ReplyToActivityWithHttpMessagesAsync Method (Microsoft.Bot.Connector) | Microsoft Docs