I just spent a lot of time trying to troubleshoot why packages wouldn't restore on my CI/CD server.
It turned out to be cause by the NuGet cache.
TLDR: Clear the NuGet cache with the following command. Note: You'll need the NuGet.exe. Command: nuget locals all -clear
I was getting an error saying C:\Program Files\dotnet\sdk\2.1.403\NuGet.targets(114,5): error NU5000: Nuspec file does not exist in package. which didn't make sense since this worked fine on my workstation.
I isolated the problem to a specific package: Microsoft.NET.Test.Sdk. I tried several version including 15.9.0, 15.9.0-preview-20180924-03, 15.8.0, 15.7.2 the latter being the most popular release.
They all gave the error which lead me to try to clear the cache. I used the command above and the issue was fixed.
Note: I don't believe the specific package had anything to do with it. It was just a symptom of a larger problem.
