I had the same problem, so I thought I would add as I solved it. I needed to install OpenCover and Nunit on my machine for OpenCover to work in VS. Installing only the vsix package for OpenCover is not going to do this. Go to the respective websites and download the msi files for each. Then run OpenCover again in VS. He had no problem finding Nunit, but OpenCover was installed in C: \ Users {yourUserName} \ AppData \ Local \ Apps \, so I had to point it there. Then it worked.
dougF source
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