Q: track pad issue

If your Apple wireless mouse, keyboard, or trackpad aren't working as expected - Apple Support

If you are using 10.11.4 you are two updates behind and should update to 10.11.6 which was released in mid July. If this doesn’t solve the problem:

There are three possibilities: 1) total coincidence and you have a hardware issue. 2) something went wrong with the update. Get the 10.11.6 combo updater which I’ve linked and install it. Sometimes a combo update will fix little hiccups that occur during an update. 3) Do you have any software that changes the behavior of your trackpad? (I use Better Trackpad Tool, for example). Your problem could be an incompatibility between a similar utility to you have installed. Check for updates to this software.

The 10.11.6 Combo Updater can be used on any version of 10.11.0 to 10.11.6 and that error message says it cannot find a valid 10.11 disk to install on. Are you sure you have 10.11 installed on that computer? Are you sure you aren't trying to install an older combo on a newer system? The combo 10.11.4 won't install over a 10.11.6 for example.

So it is either hardware or some utility that needs to be updated.