You don't get the potion directly from the guy in the lab, but loot the place after he leaves.
The link Lady Rain posted goes into more detail, but do you have the Quetzalcellona mushroom that Nixfix will trade for the root? Try dropping it on the ground away from him somewhere and have your hero pick it up again, and then talk to Nixfix. Did you go through all of the speech options? If completing the other quests first got rid of the speech option to trade, you can download the
inventory files to get a replacement root.