Hey there and welcome to the Samsung UK and EU Community Forum
When you set up your device, before you can set up a fingerprint you need to create a pin. Every so often your device will ask for your pin instead of your fingerprint. This is a security feature and works a bit like contactless card payments using a debit card, every so often you need to enter your pin.
I am not employed by Samsung nor do I receive any remuneration as a Samsung Members Star.
My device is an S25 Ultra.