Hey @Kiera ,
let's have a look into this together. Would you be able to run a Hardware test to determine if this is a hardware fault? You can do this from the Samsung Members app> Get help> Interactive checks> Fingerprint. Is this displaying as normal after running the test?
That indicates that there is an issue with the fingerprint scanner and the unit will need to be examined by one of our engineers. Please contact our Support Team to get this arranged: https://www.samsung.com/uk/support/service-centre/