I had this issue and I've since resolved it without factory resetting. My phone is stock Android P with bootloader locked still.I can't use google pay on DP4. I upgraded from DP3 (it worked there) and made a factory reset. I'm not rooted or unlocked.
Does anybody have the same issue?
Clear storage from your Google Play Store. Shortly later (or instantaneously), you should see your Device Certification being reflected as "Certified" when you go into Settings from within Google Play Store. Google Pay should work now.
---------- Post added at 11:53 AM ---------- Previous post was at 11:51 AM ----------
Try clearing storage from Google Play Store. It should work now. See my previous post.I have the exact same issue, no problem on DP3, had to factory reset on DP4 and lost google pay. Its disappointing.