You create a PIN as a security step. The Yoti app uses PIN authentication as its primary authentication method. In addition to PIN you can also choose to use your fingerprint.
Select that option, when you create your account, and you will be able to access your Yoti using your fingerprint instead of the PIN.
If, by mistake, you deactivate it, you can set it up again by going to Settings > Account Settings > Log In Settigs.
In order to use it you will need to have the Fingerprint log in activated on your device as well.