Editing "Image Target Behaviour (Script)" is not possible!

I am using Vuforia Engine for AR development in Unity since 2017. Never have come across this warning. Though it is a warning only but what it does is that it is not allowing to launch Visual Studio when i try to Edit my "Image Target Behaviour (Script)". It says "Unable to open Packages/com.ptc.vuforia.engine/Vuforia/Plugins/Managed/iOS/Vuforia.UnityExtensions.dll: Check external application preferences." It is showing the same message on all of my projects where I was able to edit the this script in VS without any problem. More interesting is the fact that other default scripts like "Default Trackable Event Handler (Script)" are getting loaded in VS without any problem. So, Any idea/help what is going wrong here? I have been struggling with this error since last 4 days now.