Did you unplug HDMI? You need to have touch screen connected and HDMI disconnected.
I’ve tried on my VIM3 Pro with this image and TS050 worked: VIM3 with Ubuntu bionic + Touch screen TS050
I am also a bit worried TS050 is being sold with little compatibility at the moment and VIM3 is actually positioned as dual display device (RPi 4 supports dual monitors via two miniHDMI ports) whereas at the moment only one of the displays work… But I trust Khadas team will fix that 