question: Unable to install ExpressVPN due to incomplete uninstall
answer: 
  This guide will show you how to resolve Windows Installer error 1721 when trying to install the ExpressVPN app:
  
  Error 1721. There is a problem with this Window Installer package. A program required for this install to complete could not be run. Contact your support personnel or package vendor. Action: Uninstall3x, location: C:\Program Files (x86)\ExpressVPN\Uninstall.exe, command: /S"_?=C:\Program Files (x86)\ExpressVPN\""
  
  If you encounter this error, you will not be able to install the ExpressVPN app.
  
  To resolve the issue:
  
  Download and open the Microsoft Install and Uninstall Troubleshooter.
  Click Uninstalling.
  Click ExpressVPN > Next.