I'm getting "[project] nuget configuration is invalid" error. I received an error like this before and used the 'Update Nuget package manager' solution mentioned here:
Unable to Install Any Package in Visual Studio 2015
I've also tried the other solutions mentioned in that link to no avail.
Also, I'm able to see the nuGet Packages at the solution level and other projects within this solution.
And, if I go to Package Manager Console (Tools -> NuGet Package Manager -> Package Manager Console), I can't see the projects having this issue in the project drop down, but I see the other projects in the solution.
Simply restarting Visual Studio worked for me.