Dubrovnik Grocery store for picnic & Cable Car

Do you mean sunset picnic? Cable car does not start running until 09

There is a Maxi Konzum closer to Lapad and a Tommy even closer to Lapad, the grocery stores near cable car (Studenac, Konzum) will be smaller and will have less choice.

Yes, it would help if you mentioned where you are located or at least where you'd
ideally like to get items.

There is also a full size Konzum in the area of the cruise ship docks.

But now that you know the names of the grocery chains in Dubrovnik (which, to
be honest, Google Maps would have also told you), you should be able to have
a go at it.

There is a small Konzum grocery store inside the old town walls on the main square.
Also, just outside the Ploće Gate there is another store and I believe a bakery. It’s a short walk to the cable car station from the Ploće Gate.