Mercari problem

Apple support is different than Mecari's customer support. However, with the troubleshooting you have done and because this only happens with Mecari's app this would not be an issue for Apple. It's an isolated incident related to Mecari, so they as the app developer is the one who would help resolve this.

You made sure you do not have any VPN's or MDM profiles installed and on, right? Your iPhone is up to date, too?

Connect to a WiFi network and try it again.

Unfortunately, since that’s not an Apple product or service, you’re going to have to work with the developer to resolve this.

What is the full error you are getting? Do you have issues getting other apps, too?