I made an in-app purchase in Sena, and it should unlock the hidden quest, right...?

If it doesn't show up, try contacting support.

I can't do the payment missions, they're too hard ㅜ🥲

If you inquire, they'll come right in.

I haven't received it yet either, how should I inquire? ㅠㅠ

First, check if you made the in-app purchase through Google. If you did, take a screenshot of the payment status on Google's side and contact support.