I was facing this issue when I added Nuget package Newtonsoft.Json 12.0.0.2 into my two .netstandard library projects and it grabbed almost my full day to solve this issue.
Exception -: Could not load file or assembly 'Newtonsoft.Json, Version=12.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed'. The system cannot find the file specified.
Solution -: I had to remove package from Nuget and went to the following location and followed the next steps -
Step 1. Go to location "C:\Users[UserName].nuget\packages\newtonsoft.json\12.0.2\lib" and here you will get all Nuget versions you installed previously.
Step 2. Since i wanted to use it in my .netstandard 2.0 library project, so i copied "netstandard2.0" folder from this location and paste somewhere my preferred location (**where i generally keep 3rd party dlls).
Step 3. Now i added DLL reference from here to my both project and this way problem solved.
Thanks