My error was as follows:

Severity    Code    Description Project File    Line    Suppression State
Error (active)  CS0006  Metadata file '..\packages\AWSSDK.S3.3.5.7.8\analyzers\dotnet\cs\AWSSDK.S3.CodeAnalysis.dll' could not be found CCNRebuild  C:\Users\jp2code\source\Workspaces\Dealers Rebuild\CCNRebuild\CCNRebuild\CSC    1   

This was after a fresh install of my project from Source Control.

For me, I went into the Package Manager (Tools > NuGet Package Manager > Manage NuGet Packages for Solution...")

I located my package in Nuget by searching for AWSSDK.S3 and picked the version that matched from the dropdown list:

It looks like the last person forgot to check in a package that they had added to the project.