Reference TweetinviAPI (.net core library) from .net Framework 4.6.1 webforms project

I am upgrading to TweetInviAPI 4.0.0 in my webforms project. As TweetInviAPI 4.0.0 is a .net Core libary I am encountering the following error:

Could not load file or assembly 'System.Runtime, Version=4.2.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

I have read that installing the NETStandard.Library.2.0.3 is a solution but this has not worked for me.