Can't 'Convince to Buy' on Food Stall

I don't think it's a bug. Some aspirations are just unreasonably complicated or illogical. One of my Sims is selling some of her food at a foodstall, too, usually before she goes to work, and convincing another Sim to buy that food doesn't always work because that Sim might walk away. Make sure you don't try to use the interaction on every Sim that approaches your stall because once it has been completed, it greys out for a while before you can use it again. It's really annoying and doesn't make much sense either, especially with high charisma.