I've been seeing the same thing with several of our organization's 2015 MBPros. Usually the Trackpad Cable needs to be replaced, but in a couple of cases we had to have the Top Case Assembly (Keyboard/Trackpad) or the Logic Board replaced. I believe the Trackpad cable becomes worn/damaged from rubbing against the frame after several years of use. You will need to arrange a repair with Apple or an Apple Authorized Service Provider or click on the "Contact Support" link at the top right of this page to see your options. In some regions it may be possible to mail the laptop to Apple for the repair.
In the mean time you may be able to continue using the laptop if you use an external USB keyboard and mouse.