All devices starting from Android 5.1 have a new security feature: device protection a.k.a. factory reset protection a.k.a "kill switch". It is meant to protect the phone from unauthorized access (phone lost or stolen). Device protection is enabled by setting screen lock and creating a Google account in the device.
If device protection is enabled, it will be not be possible to access the device, and thus it will not be possible to erase the device with Blancco 5 Mobile. Device can by unlocked by providing the Google account credentials of the previous owner.