Earlier today, it was reported that the March 2017 security update for the Nexus 6 had been pulled by Google. However, it now seems as though the March security update has been made available again for Nexus 6 owners to download. Although, it is currently clear if anything has changed with the the update, as it seems to be sporting the same build number as before. So this is not thought to be a different update.
The reason it was initially pulled seemed to be due to a number of users becoming affected by an Android Pay issue after installing the update. Specifically, the Android 7.1.1 update. According to the details, it was actually the SafetyNet API that seemed to be causing a headache for users, as it is believed to have been disabling Android Pay on updated Nexus 6 devices, without any justifiable or logical reason. Or if not disabling, then at the very least, somehow stopping Android Pay from functioning correctly. At the time, Google's Nexus account on social media had been noted responding to questions from users and advising them that they were aware of the reported issue and that they were investigating it. Although since then, there does not seem to have been any update from Google on this and therefore, no formal confirmation has come through on what the issue was. Likewise, whether the update has been changed, SafetyNet disabled, or a patch applied. Due to the speed of the resubmitting of the update, it could be the case that only a quick fix has been applied to tide the update over while the real issue is addressed.
However, as the update is now back again, it is presumably good to go for owners of the Nexus 6 and after installation, everything should be working as normal, including Android Pay. Head through the link below if you do want to grab the resubmitted Nexus 6 Android 7.1.1 March 2017 security update (build number N6F26U). For reference, it was only the Nexus 6 that seemed to be suffering from this issue, as there were no reported issues with the March security updates for the other Nexus and Pixel smartphones.