It never worked on the S6/edge or Note 5, last time it worked was on the S5. It stopped working after a Paypal update and not even tons of complaints have forced them to work on it.
It must be a Marshmallow compatibility issue. I have the latest Paypal update on my S6 and fingerprint login works fine. However, on my S7 Edge, there isn't even an option to configure it. It's missing from the settings. Hopefully Paypal updates their app.