I have also had a few problems with samsung pay on my old phone a few years ago. I didn't really bother and after a update the problem was gone.
I think you should try to see if you can update the app or your phone.
For the app open Google play store and search for Samsung Pay and see if it needs updating.
And for the phone open settings, scroll down to software update click on it then click on download and install, the phone will check for any updates, if there are any then update the phone.
I hope this helps.