I did both of them.
- Settings -> Update & Security
- Click 'Update now' of https://www.microsoft.com/en-us/software-download/windows10 and execute
Windows10Upgrade9252.exe
There are several users have the same issue.
- https://twitter.com/GlynPress/status/1067486762593910790
- https://answers.microsoft.com/en-us/surface/forum/all/surface-pro-6-wont-upgrade-to-windows-10-1809/8bb7c903-cb1c-4391-a8fc-5858caa48651
Is not this update pushed to Surface Pro 6 yet?