I've been unable to update Samsung Pay for a few months since I joined the nougat beta. Even after I installed the official update, I still get an update failed message.
I'm not on the beta but my phone has already been updated to Nougat and don't have issues updating SP. Have you tried uninstalling and re-installing? You'll probably have to re-add your cards (loyalty ones are stored with your account, though) but that might clear any issues.
Glad it helped! Not sure why, but when I had one of the Note 7's, I couldn't make any payments half of the time... I uninstalled, re-installed and voilá! No payments failed. Weird.