I had the same problem, even with no other errors showing on the "Error List" view after "Rebuild Solution". However, on the "Output" view, I saw the error that was behind the issue:
The primary reference "C:...\myproj.dll" could not be resolved because it was built against the ".NETFramework,Version=v4.6.1" framework. This is a higher version than the currently targeted framework ".NETFramework,Version=v4.5"
Once I corrected this, the issue was resolved.