well it's been a while I played this game and honestly almost forgot everything, but as far as I remember you also needed top quality plant or something like that. the green/blue capsule
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Another late reply - after acquiring the required gold from the arcade machine in the downtown sticker shop, you gotta click your white character icon on the right, scroll down to the skill list and click on the new skill - it will allow you to use the gold and update the skill. After that, it's similar to other skills, you have to keep increasing proficiency for next levels.