@ArchAngel1294
Yes, I meant ice cream my sim made. (And watch out - you cannot sell any ice cream that has been eaten by your sim - not even one bite. And I think with platters of prepared food or sets of baked goods, the whole serving dish must not be eaten from - all 8 servings have to be there. Apparently not with ice cream, though, since you cannot sell it as a whole? (At least I couldn't.) And watch out for ice cream cones. My sim kept grabbing the cone and taking a bite when I had them grab a cone from the carton - so once they bit from it I couldn't sell it.
Also, to your question... whether you have a whole carton or individual ice cream serving in your store, you should be able to 'Set for Sale' in the options when you click on the item. If you're not seeing this you may want to run a game repair if you have just purchased the game recently. Perhaps it didn't fully download properly. Open Origin, click ‘Game Library’ then right-click Sims 4 > Repair Game. If you're on PC wait until all packs have run. If you're on mac, it will only repair the base game. You can get directions for repairing the packs at this link: https://bluebellflora.com/2020/06/17/repairing-sims-4-packs-in-origin/
Or if you have Mods or CC they may be causing the issue of not being able to 'Set for Sale'. If this may be your issue, let me know and we could trouble shoot that.
I just went into the same store. I went this time with a different sim from a different household. The employee showed up and other customers, but not the owners. She was able to purchase the ice cream plus other items from the freezers. (I believe in this case the game was running the store.)
I then took my owner sim to the store, which was not open. No employee showed up, and no other customers showed up. Through her I only got the option to open the freezer. If I dragged an ice cream bowl to the counter, then 'marked it not for sale', then she had the option of eating the ice cream.