My rough steps for getting Thinkpad P50's Fingerprint sensor working on Pop OS 20.04. I also wrote Thinkpad P50 Fingerprint sensor on Debian 12.

Install snap.

sudo apt update
sudo apt install snapd

Install validity-sensors-tools and configure the sensor

sudo snap install validity-sensors-tools

# Give it access to the usb devices
sudo snap connect validity-sensors-tools:raw-usb
sudo snap connect validity-sensors-tools:hardware-observe

# Initialize the device
sudo validity-sensors-tools.initializer

# Test the device
sudo validity-sensors-tools.led-test

# This is needed and only works in 138a:0097:
sudo validity-sensors-tools.enroll --finger-id [0-9]

# See other available tools
validity-sensors-tools --help

Use custom libfprint from here.

sudo add-apt-repository -u ppa:3v1n0/libfprint-vfs0090
sudo apt install libfprint-2-tod-vfs0090

Install pam-fprint and friends

sudo apt install libpam-fprintd pam-auth-update

# enable fingerprint login
sudo pam-auth-update
# (terminal gui opens, select fingerprint sensor)

Then go to the user settings and enable fingerprint login.