Police can't force you to unlock phone using fingerprint
Compelling to unlock the device using fingerprint would be self-incrimination.
In the US district of Idaho a federal magistrate judge denied issuing a search warrant authorising the police to compel an individual to use their fingerprint for unlocking their device.
As stated in the official document, the case relates to a Google Pixel 3 XL unit which is part of other electronic possessions of an individual suspected to be involved in child pornography.
The document further notes that search and seizure are unlawful and unreasonable when it violates an individual's constitutional rights. Compelling to unlock the device using fingerprint would be self-incrimination, communicating the ownership of the device.