Hmm... well this is kinda funky...
I've done this quest before (about 3-4 months ago I believe), and I followed the recipe (1 Berries + 1 Water Flask + 1 Bottle), and after a couple of attempts, I succeeded in making Berry Juice.
However, trying the combine today, I got the system feedback message: You cannot combine these items in this container type!
So I tried a few things (2 Berries, no Bottle)... and I was successful in combining just 1 Berry + 1 Water Flask (and no Bottle), to make Berry Juice.