DEACTIVATE RESET CHECK BOX IN THIN OS 8.6

ThinOS 8.6 needs to be "Managed" in order to change all options. 

Legacy it used INI files

https://dl.dell.com/content/manual17268497-dell-wyse-thinos-version-8-6-ini-reference-guide.pdf?language=en-us

Privilege mode is on page 52

If using WMS, it is in a policy,

I am guessing you have a second hand device that you cannot make changes to and you want to reset it. 

https://crsi.zendesk.com/hc/en-us/articles/360048736174-Factory-Reset-a-Dell-Wyse-Thin-Client

If you cannot do a G Key, your only choice is to reimage the device using a USB Flash drive.