I just started with Selenium and C# and came across this problem at very beginning. I've installed nuget webdriver package in project WebdriverTest, then created Unit test project in which I add reference to WebdriverTest project. But the unit test doesn't recognize the selenium webdriver reference in WebdriverTest project at all.
The project reference for "WebdirverTest" in the UnitTest project's references does not include any libraries that are in "WebdirverTest", it only includes references to classes within that project. You must add the library to both projects. As @ChathuD said, the Nuget Package Manager is a useful tool for doing so.