Regarding the Issue with Unlocking the "Special promotion" Achievement — Together with Oneesan~Yuina's Sweet Encouragement~ Steam update #401056
If you are unable to unlock the "Special promotion" achievement, please make sure you have purchased the aquarium doll. How to purchase the doll: Watch...
If you are unable to unlock the "Special promotion" achievement, please make sure you have purchased the aquarium doll. How to purchase the doll: Watch all the dialogue events at the aquarium. After that, the doll purchase option will become available at the shopping mall.