Well, as the screenshot says - you should already be able to unlock your phone via this stored fingerprint.
I’d propose to add your finger again as additional fingerprint. Maybe the device does no longer recognize the first finger already stored?
Apart from that there had always been lot of reports in the past about the FP3 fingerprint sensor not working perfectly.
Maybe you have a look at this post:
Or at this topic:
And (even as it sounds starnge) for some it has helped to put an adhesive tape (“Tesafilm”) over the fingerprint sensor to improve it.