Impossible de finaliser un achat
Au secours, mon achat n'a pas pu être effectué
iPhone XR, iOS 18
Have a look at the related troubleshooting mentioned in this article:
If your payment method is declined in the App Store or iTunes Store - Apple Support
Si votre mode de paiement est refusé dans l’App Store ou l’iTunes Store - Assistance Apple (FR)
Or have a look at this related discussion:
Your purchase could not be completed
Did you know that there is also a French speaking Apple Community?
You might get more answers in these forums from native speakers.
Communauté d’assistance Apple officielle