I'm not sure what the countertop is made from, but it's a one-piece solid surface. It's not anything like quartz or granite, and many years ago it was chipped. Is there some kind of repair kit I can use to patch the missing chunk? I don't have the missing chunk.
Solid surface counter tops are typically seamed with epoxy mixed with the powdered countertop material. The best thing here is probably an epoxy mixed with a stone powder that matches as close as possible. After patching you'd then have to polish. Diamond abrasive pads work well for that you need some 800-5000 grit pads. You are unlikely to get it to look perfect or not be able to notice it though but it will certainly look better.
Natural stone is often patched with epoxy where there are natural voids in the stone.
