I have recently started looking into Android Pie Beta for my S9+ Verizon. What I believe is that my Oreo build (3ARIB) is not the correct build to install the newly leaked Beta Pie builds. As referenced earlier Verizon doesn't appear to want to participate in beta testing (every other major carrier/variant of the S9/+ is supported) just not verizon. That being said I have installed an Alpha version then went back to stock ROM and my device is still official and knox is not tripped. So for now I'll wait and keep checking here to see if my Verizon S9+ will get an early slice of Pie.
What file did you download the zrb7 or 8? For the alpha pie I was thinking about installing bit and another question how do I go back to ar1b with Odin? I've never used it before