If this is on screen, try to remove any screen protector you're currently using.
Even if Samsung has a feature that increases sensitivity to screen accessories, ideally you'd want to set this up without it.
Normally what I do is, I would register the same fingerprint 2x for each to in a way, increase the chances of it being recognized.
If however this is a side-mounted physical fingerprint scanner, and it isn't being detected, best to bring this to an authorized service center.
Don't try to look for quick fixes online as most often than not, whatever fixed their issue may not apply to yours, and could just lead to more possible problems.
