'MonoBehaviour' is defined in an assembly that is not referenced

So working with visual studio is fun and all but I am running into a error which is irrelevant in a way. The error VS is throwing me does not affect how my plugin runs so much, but how VS debugs.

Code:

Severity    Code    Description    Project    File    Line    Suppression State
Error    CS0012    The type 'MonoBehaviour' is defined in an assembly that is not referenced. You must add a reference to assembly 'UnityEngine, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.    HumanitySystem    c:\users\***\onedrive\documents\visual studio 2015\Projects\HumanitySystem\HumanitySystem\Class1.cs    120    Active

Any idea's on how to remove this error? Here are some examples of it.
*Anything underlined in red ` marks are where the errors are occurring(same error every line).

Screenshot