Why does the native plugin sample not work in 5.5.3?

Trying to write a native desktop plugin for windows and getting DLLNotFoundException (not much detail there to work with ).

I then tried the unity project from the official sample provided at this link from https://docs.unity3d.com/Manual/PluginsForDesktop.html and have the same error.

(The first error was that the sample seems to be x86 and by default unity looks for x64, but having changed this to only be x86, it then continues to the DLLNotFoundException error.

Anyone seen this, clues as to why this does not seem to work?