The option to give consent in the UI has been added with NuGet Package Manager 1.8. It’s the 3rd item under Tools-> Options-> Package Manager-> General : “Package Restore : Allow NuGet to download missing packages during build”.
Please make sure NuGet is up-to-date in Tools->Extension Manager.
Alternatively you can give consent by setting the EnableNuGetPackageRestore
Environment variable to true
.