I have a XPS 15 9560.  I attempted to start SupportAssist today and was unsuccessful.  I looked at several approaches to solve the problem and chose to follow these instructions:

https://www.dell.com/community/SupportAssist-for-PCs/SA-uninstall-reinstall/m-p/6056368#M84385 

Every time i try to reinstall it fails and advises that I restart my computer and try again.  Help!!!