I'm running into this issue as well. I tried to install the dependencies separately and then retrying the Tweetinvi install.
This worked for Autofac, Newtonsoft.json and Microsoft.Bcl.Async, but NOT for PCLWebUtility (1.0.2)
This worked for Autofac, Newtonsoft.json and Microsoft.Bcl.Async, but NOT for PCLWebUtility (1.0.2)