Ever since the March security update for the Motorola Nexus 6 was made available, several users have been reporting that they are unable to use Android Pay after installing the update.
Google recently confirmed the issue in reply to a user query on Twitter, saying they are investigating the problem.
@MeKevinB Hi Kevin. We're aware of this issue and our team is investigating. Thanks for your patience!— Nexus (@googlenexus) March 9, 2017
The update was temporarily pulled. Although the factory image and OTA file for the update have been uploaded again, the company is said to have disabled SafetyNet API for now - the API was affected by the March update (which in turn broke Android Pay).