bool? CanSetRelationship(BasePlayer player, BasePlayer otherPlayer, RelationshipType relationshipType, int weight)
{
    Puts("CanSetRelationship works!");
    return null;
}​

error CS0246: The type or namespace name `RelationshipType' could not be found. Are you missing an assembly reference?

I'd recommend getting familiar with a .NET decompiler. Open the Assembly-CSharp.dll in a .NET decompiler such as dotPeek, JustDecompile, CodemerxDecompile, etc. and look for that type, RelationshipType. You'll find that it is under RelationshipManager in the global namespace, so you can just add "RelationshipManager" before RelationshipType so that it'd be RelationshipManager.RelationshipType.

why not document it as such?

		private bool? CanSetRelationship(BasePlayer player, BasePlayer otherPlayer, RelationshipManager.RelationshipType relationshipType, int weight)
​

Adding using statements and references is something that your IDE would help you with when setup properly. The above is just an example to help point you in the right direction.