Resetting Nexus 6 requires previous account
This is an anti-theft policy put in place by Google. There is not supposed to be any way to use the phone unless you have access to a prior account used on the device. See the Nexus documentation.

The user with access to a prior account should remove that user to allow future access: 


  To turn off device protection, remove your Google account from your device. If you have Developer options enabled on your device, you can also go to Settings Settings > Developer options and check "OEM Unlocking."