cannot syn ipad 5th gen to Mac Air 2014 running High Sierra

OK, thanks. You may have already worked through all of the steps in this Apple support article:

If your computer doesn't recognize your iPhone or iPad - Apple Support

but it's worth checking.

Regards.

Are you trying to sync via WiFi, or are you connecting the devices via a cable? If via WiFi, has this worked before or is this the first time you're trying to sync the two?