This stems as far back as late July in early PTS 7 build as seen here - https://forums.focus-home.com/topic/52898/customization-menu-bug-pts-7-2
I don't understand how this never got fixed, yet got broken somehow in that build.
This has been fixed as of 10.1 - https://forums.focus-home.com/topic/52898/fixed-customization-menu-bug-pts-7-2-international-transtar/14
I also just fired up the current 10.2 PTS and the snorkel can be purchased/installed, unequipped or sold without issue on a newly purchased truck.
Now I didn't have one already in my storage when I transferred my live save over so if you already own one and still see this error, you should sell and repurchase the truck similar to a recent update which certain White Western Star's equipped with the old heavy crane required selling and repurchasing to correct the "duplicate cranes".