Android 5.1 devices have a feature called Device Protection which requires Google credentials to be entered before and after a device can be reset. When a device is ready to be enrolled as a Work Managed device for Android for Work, the device must be factory reset.
Any existing Google account needs to be removed from the device and the secure lock screen disabled to avoid triggering Device Protection so that the AirWatch Agent can be installed during enrollment. This also prevents the new user from being locked out of the device.
In the event the previous owner changed the Google account password, you must wait three days before factory resetting any of your Android 5.1+ devices for enrollment unless you have explicitly disabled Android Device Protection on them. If you factory reset one of your Android devices before those 3 days are up and then attempt to sign into that device with your Google account, you will be met with an error message and not allowed to log into the device with any account until 72 hours after the password reset occurred.