When installing the package on Umbraco 9, you need to use the NuGet package.
The package can be found on NuGet.
Just install the package using the Package Manager inside Visual Studio or via the console.
dotnet add package NewsletterStudio
We host several packages on NuGet:
Since Umbraco 9 don't support package installation from the backoffice this not supported by v4+ of Newsletter Studio.