You have a few option , go back to your installer and ask him to fix the problem or suggestions as to what he can offer to fix the problem . Or , You can put a molding , or cover the entire left side with a piece of wood in order to make the left wall thicker and filling the space .

I would not recommend your “millworker” to anyone. The gap must be covered by a new panel. Have him show you what it will look like before it is permanently installed. It looks like he used the wrong end panels to begin with.

Millworker? I don’t think so and sure not one I would hire.IMO that whole end look odd maybe post a pic further back.

Put a panel with a finished edge over this entire area unless your "millworker" will remake the whole magilla.

Facepalm fail. Find someone actually qualified to work with cabinets. Part of this is the design fail too.

Looks bad. Not enough countetop overhang on the end for a plant-on panel. Why is the end panel out into the seating area to the countertop edge also? It's not thick enough for the column look. You can notch a piece of matching scribe molding to fit the backside profile of the panel for poor fix. Fire the milworker and get someone qualified!