I'm still in the U.S. and am going to Paris next week. I've downloaded and installed the IDF app, but when I try to purchase a Navigo Decouverte weekly pass (Forfait Navigo Semaine) by first creating an account, the app throws an Error 15 ("This request was blocked by our security service.") Can anyone please help? Do I need to be in France to make it work?
For the Navigo week pass purchases via the Ile-de-France Mobilites app (or RATP Bonjour app: the error 15 you're seeing is perhaps related to systems issues on the RATP/IDF side and not actually an error on your phone. There have been several complaints on reviews of the Android / iPhone apps detailing similar purchasing issues related to Navigo cards.
To buy & use a Navigo Découverte pass directly from a supported Android phone without a physical plastic Navigo card you'll need an Ile-de-France Mobilites Connect account created before purchasing. You'll have to supply a face photo and date of birth to be stored in the app / on your Connect account to complete the Navigo purchase on phone (see section 4.3 on this page). Google Pixel 4 and earlier Google phones do not support storing a Navigo card on the phone itself. Most other Android phones with Android 8.0 and above, with NFC support, are "theoretically" supported. Samsung phones have the most "official" support for storing & using Navigo directly from phones. iPhones are not supported yet as of 2023. See the section: "Comment charger des titres dans votre téléphone et valider avec ?" on this page for more information supported phones.