Reinstall the VCOM drivers or try a different USB port/cable.

Click the button. Turn off your MT6877 device. Hold the Volume Up + Volume Down buttons and connect it to the PC. The progress bar will turn red, then yellow as flashing begins. 5. Success

This open-source tool allows you to bypass the security handshake (Auth Bypass). It lets you read/write partitions without needing a "DA file" (Download Agent). Professional Service Tools (Dongles)