Yes, the way Samsung designed Samsung pay is to make it as secure as possible, and unfortunately that includes blocking access on modded devices. Is there a way around this? I belive it would have to involve tricking Knox and I don't think it will be easy.
With that said, the galaxy s6 had a root that didn't trip knox. Even still, had to be extremely careful after that with flashing roms and such but it was b possible.
It's really the only thing that's going to make me think twice about rooting.
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