Google has introduced a new security feature starting with Android version 5.1, which should protect smartphones even if they are stolen and the thief performs a factory reset.
In this case, all data is normally deleted from the device and the smartphone is reset to the factory default settings. However, to prevent unauthorized access to the smartphone in this way, Google has introduced the so-called "Android Factory Reset Protection" (FRP). This means that after a factory reset, a login with the Google account last used on the device is required before the smartphone can be used again.
Thus, a factory reset is of little use to a thief, as the device is still not usable after the factory reset.
The last known Google account is mandatory. The smartphone is only unlocked again when this has been entered completely.
If the security message appears and the security code or the Google account is no longer known, you only have the option of restoring your own access data via Google.
Link to Google Support.
Alternatively, your smartphone can be unlocked in our service workshop for a fee. Please contact our service team for this.