Could not resolve this reference. Could not locate the assembly

If anyone face this issue with some nuget packages, you can fix that by reinstalling the packages using the Package Manager Console:

Update-Package -reinstall 

You most likely get this message when the project points to an old location of the assembly where it no longer exists. Since you were able to build it once, the assembly has already been copied into your bin\Debug / bin\Release folders so your project can still find a copy.

If you open the references node of the project in your solution explorer, there should be a yellow icon next to the reference. Remove the reference and add it again from the correct location.

If you want to know the location it was referenced from, you'd have to open the .csproj file in a text editor and look for the HintPath for that assembly - the IDE for some reason does not show this information.