You may encounter this error while you upgrade or new install NetScaler VPN plug-in
Error message "The older version of NetScaler Gateway Plug-in cannot be removed."



SSLVPN logs
nsverctl.txt
===============
ns_delregvalue | 215 | Failed to delete modifiedRoutes registry value, err 2
CnsServer::nsServer_regapi | 855 | Failed DeleteRegValue for 'modifiedRoutes'
ns_delregvalue | 215 | Failed to delete addedRoutes registry value, err 2
CnsServer::nsServer_regapi | 855 | Failed DeleteRegValue for 'addedRoutes'

~nssslvpn.txt
===============
 | ERROR   | ns_delregvalue | 215 | Failed to delete modifiedRoutes registry value, err 2
 | DEBUG   | Deleting 'modifiedRoutes' registry key failed
 | ERROR   | ns_delregvalue | 215 | Failed to delete addedRoutes registry value, err 2
 | DEBUG   | Deleting 'addedRoutes' registry key failed


You may have tried cleanup tool in below KB and find the result is the same , issue still remains.
https://support.citrix.com/article/CTX122969
https://support.citrix.com/article/CTX137494
 

Microsoft removal tool is needed:
https://support.microsoft.com/en-us/help/17588/fix-problems-that-block-programs-from-being-installed-or-removed

This will help us to clean all registry of older version NetScaler Gateway Plug-in.
1.Download the tool which name is " MicrosoftProgram_Install_and_Uninstall.meta.diagcab " and size only 216KB
2.Run the tool and select "Uninstall" --> "NetScaler Gateway Plug-in"
3.After clean left registry of old version, you could install new version of NetScaler Gateway Plug-in successfully.