Error CS0006: Metadata file 'Assembly-CSharp.dll' could not be found (CS0006) (Assembly-CSharp-Editor)

I was going through the Survival Shooter tutorial and I get this error with both my script and the CompletedAssets one.

I have no idea what it means and google doesn’t seem to know either.

What do I do?

I had the same issue… But closing monodevelop (if you use that) and then restart the monodev again solved my issue of the same error.