On that board you should be able to flash firmware over telem1/uart7. That method will almost certainly fail if you try it over a radio, but if you connect a serial/USB adapter (ftdi), it should flash fine. For anyone else reading this or encountering a similar problem, check the bootloader hwdef for your board and see if a serial port is enabled in it (ie https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_HAL_ChibiOS/hwdef/MatekF765-Wing/hwdef-bl.dat)