Unable to flash Arduino when TXD pin of HC-6 Bluetooth module is connected to Arduino RXD pin?
Most* Arduino devices are flashed through a bootloader which communicates over the serial pins. If you've connected another device to those pins, that communication may fail. You may need to disconnect the device while you flash the Arduino.

(*: The exceptions are the 32u4-based devices, like the Leonardo and Micro. These communicate directly over USB.)