I am not with Laminar Research; just a flight simmer.
Firstly X-Plane 11.30 has been designed primarily around Windows 10 so to upgrade from Win 8.1 would be a prudent move.
Secondly what is SimBuddy and why are you using it. Either delete the software or isolate the product so that it has no impact of any sort on the installation X-Plane. X-Plane requires a complete greenfields approach with no other software influences.
Thirdly X-Plane needs to be installed to your desktop or a dedicated created directory on your "C" drive or on your "F" drive. It has to be an isolated package that is not installed in your windows operating system files like other older style flight packages.
Fourthly if you have a digital download version you have to be connected to the internet at all times as the software does a silent background check as you use X-Plane to ensure you have a legal copy. So a move to upgrade your internet plan is also a good move.
My suggestion is to use a good reliable software deletion package and remove any traces of X-Plane and SimmBuddy and after you have upgraded your internet plan then reload and reinstall X-Plane.
As a side issue you haven't indicated whether you have the official version of X-Plane from Laminar Research or the 3rd party hybrid version from steam. The steam version and the Laminar version are totally different packages. They do not mix and match.
Good luck with the new installation.