[Red Badger]

Fingerprint ID on iphone doesn’t work after it hasn’t been used for a couple of days.

Why?..................

[I Drive Too Fast]

From Apple.

To start using Touch ID, you must first set up a passcode on your iPhone or iPad (or a password on your Mac). You must enter your passcode or password for additional security validation:

after you restart your iPhone, iPad, or Mac;

when more than 48 hours have passed from the last time you unlocked your device;

[DesertRhino]

“Fingerprint ID on iphone doesn’t work after it hasn’t been used for a couple of days.”

After a couple of days it is rigged to require a password. Also a dead finder won’t work. Those are protections from exactly this kinda thing and other government intrusions.
Personally, I think it would be a bad idea to ever use a fingerprint.