Pay Android seems to not work on devices with boot managers released.
Android users enjoy personalizing their devices and fiddling at a much deeper level than others, chances are they have released the boot loader (known in English as bootloader) of your device. This is really the only way to get a deeper level of customization and allows us to install applications and changes that require a higher level of access. However, unless you know what you're doing, releasing the bootloader of the device can cause security problems, and therefore it has been recently discovered that Google has made some changes in its beta version of Android 7.1 to Android Pay not work on devices with a boot manager released or have been Rooted.
ANDROID PAY CAN NOT BE USED IN THIS DEVICE.
GOOGLE CAN NOT CONFIRM YOUR DEVICE OR THE SOFTWARE IN HIM BE COMPATIBLE WITH ANDROID. PERHAPS DEBAS DES-Rooting YOUR DEVICE. "
Google tries to Android Pay as safe as possible
The reason for this change, as has been speculated, might be that Google is trying to Android Pay as safe as possible; and devices with a boot manager released and whose users have access to Android super user could try to figure out how the system works and create hacks that could potentially steal information from credit cards of other users.
That said, since Android Police also suggest that it could be a mistake. They are based on input Reddit, where users who complained about the problem reported that Google had told them to report it as a bug, meaning it may not have been intentional. In any case, since Nougat Android 7.1 is still in beta, we will have to wait to see if this problem is then maintained its official release; in which case, we can already assume that something is purposefully implemented by Google as a security measure.