不平凡，就不是生活

1. Open package manager console in Visual Studio
2. Type: [Net.ServicePointManager]::SecurityProtocol = [Net.ServicePointManager]::SecurityProtocol -bOR [Net.SecurityProtocolType]::Tls12
3. press "Enter", succeed.
4. retry NuGet Manager

This will change the connection security protocol to nuget as specified in this article: Deprecating TLS 1.0 and 1.1 on NuGet.org

Reference: StackOverFlow Article