A lot of people are having issues with this particular sidequest, as I once had and opted to look on youtube for a vid-guide on how to complete it. Thankfully I found the vid-guide I was looking for and it showed a somewhat-elusive "indoor bar" section that requires some doing to find, but basically you need to look for this area in the underground mall:
Once you find it, simply go through the doors that the MC is facing in the pic and you'll be able to enter said bar room. Talk to the bartender to get the last elusive cherry ingredient. you're welcome. ^_-
(the bartender is the girl on the left-most side behind the bar counter obviously)
