Press and hold both the and Volume Down buttons simultaneously.
To successfully flash this firmware, you generally need specialized professional software or "dongles" designed for Qualcomm devices: UFI Android Toolbox oppo a57 cph1701ex-11-a.35 flash file
This error usually happens when the connection drops. Try switching to a different USB port (preferably a USB 2.0 port on the back of your computer motherboard) or use a different USB data cable. Press and hold both the and Volume Down