Invoicing app creation

I am trying to create an invoicing app buth i am blocket at the section-finalizing-the-connection with this message as response

+“success”: false +“error”: “unauthorized access” +“errorCode”: 401 +“error_info”: “Please check

Can you describe the error and use case in detail?
We also have a sample app that has the OAuth flow already setup GitHub - pipedrive/FiftyFifty: Example app for Pipedrive Marketplace

Feel free to create a new topic if you are still facing the challenge