The current .NET SDK does not support targeting …

Error NETSDK1045: .NET Standard 2.1, .NET Core 3.0

I just installed .NET Core 3.0, and began trying it. However, running nuget restore on my sln give the errors:

error NETSDK1045: The current .NET SDK does not support targeting .NET Standard 2.1
error NETSDK1045: The current .NET SDK does not support targeting .NET Core 3.0

To fix,

• Check that you have dotnet core 3.0 installed with dotnet --list-sdks
• If you do, create a global.json file in the root of your project(s) directory:

{
  "sdk": {
    "version": "3.0.100"
  }
}

Enjoy!