Raspberry pi 7" inch touch screen not working. How do I enable it?

Hello,

I installed Ubuntu mate on Raaspberry pi 3+ which is connected to a 7" inch touch screen display. The mouse and keyboard work fine but the touch on the touch screen does not work. How do I enable touch function? I have the touch screen HDMI connected to raspberry pi HDMI output. Please let me know.

Could some one please tell me how to enable touch screen in ubuntu mate?