Why is the namespace not connected to the script through using?

Question

C++ / C#

I added several NuGet packages to my project and I'm trying to use them by declaring using MailKit at the beginning; for example.
But I get the error "Could not find type or namespace name..."
Why does this error occur and how can I fix it?

Answer 1

Check that the target framework of the project fits the requirements of the package.

Answer 2

This package (Google.Apis.Auth) needs to be downloaded via NuGet. (requires .NET Framework 4.5+)

Answer 3

I had to reboot the studio