Google lately pushed out the Android 7.1.1 update to it's Nexus 6 units, but some issues came along with the update which has broke the Android Pay. Google acknowledged the issue but it seems the aren't able to find an easy solution that's why they have started pushing a downgrade OTA udpate.
Following is the reply a Google community manager posted on Reddit thread discussing the new 7.0 update:
"Hey All, There was a recent OTA update for Android 7.0 that is causing confusion for some Nexus 6 users. For those of you that want to guarantee future OTA updates, you will need to get back on the 7.0 supported track. If you are experiencing issues after accepting the 7.0 update, please factory reset your device to help resolve any problems. For any Nexus 6 users that want to continue flashing/sideloading future builds, you can stay on 7.1.1 and continue to do so. Hope this clarifies! Nexus Community Manager."