Getting 403 forbidden access is denied after adding reference to another project in same solution [closed]

I need to add a reference to a new C# class library project (which using GrapQl Client to make some GraphQL API calls) into our existing C# solution, everything works fine locally from Visual Studio. But after deployed to our test server and trying to access the site, got server error: 403 – Forbidden: Access is denied.

Any suggestions?

Much appreciated!