app store

when i try downloading a free game, it lets me use my finger print, says done, then a notification pops up saying "verification required" so i sign in, and then it takes me to "manage payments" and says that my payment failed. why are they trying to charge me for a free app? thankfully i don't have money in my account because i would've been charged. a while ago i noticed when downloading an app, after doing my fingerprint, it says "processing payment." i figured this was just an update from apple and didn't actually mean i was paying to download free apps, but now i'm wondering if they've charged me for every free app I've downloaded since then.