Projects within a solution use different versions of the same nuget package, consolidate them!

Right click on the solution and choose

Manage NuGet Packages for Solution...

This brings you to a page with tabs: Browse, Installed, Updates and Consolidate

The "Consolidate" page is super handy. It lists any packages which have inconsistent versions across projects.

consolidate_nuget_packages

Click the Install button in the lower right to bring them all into line, on the latest version. (Or pick a specific version before pressing Install)