To solve this issue, you'll need to configure your ASP.NET Core Web API to authenticate with Azure AD and make authorized requests to the Microsoft Graph API. I have detailed a step-by-step guide on creating Azure resources and managing authentication in my Medium article, which also includes practical examples for setting up and using the Azure AD access token effectively.

You can find the full solution here: Integrating ASP.NET Core Web API with Microsoft Graph API in .NET 6.0

This article provides a comprehensive explanation, including code snippets and troubleshooting tips. Feel free to check it out and let me know if you have any questions or feedback!