So, one of the early quests is to catch a Dinho. Sadly, that's one of the first Moga that I caught, so I thought -- fine, I'll release the one I have, and catch a different one.
I went to the 'team' page, and clicked on Dinho, because I'd remembered seeing a button to release the Moga when assembling a team before. I clicked on it -- and it told me that I didn't have an evolution potion.
I wasn't trying to evolve the Moga. I hadn't clicked the (clearly visible) icon with an evolution potion. I just wanted to get rid of the Dinho that I already had so that I could capture another one without crowding the yard. (Also, why doesn't my previously caught Dinho count? I wish the game could account for that kind of thing.)