Canon EOS 6D Instruction Manual page 361

The flash does not fire.
If you use a non-Canon flash unit with Live View shooting, set
[A2: Silent LV shoot.] to [Disable] (p.190).
The flash always fires at full output.
If you use a flash unit other than an EX-series Speedlite, the flash
will always be fired at full output (p.171).
When the [Flash metering mode] flash Custom Function is set to
[TTL ] (autoflash), the flash will always be fired at full output
(p.177).
Flash exposure compensation cannot be set.
If flash exposure compensation has already been set with the
Speedlite, flash exposure compensation cannot be set with the
camera. When the Speedlite's flash exposure compensation is
canceled (set to 0), flash exposure compensation can be set with
the camera.
High-speed sync cannot be set in the <f> mode.
Under [z2: External Speedlite control], set [Flash sync. speed
in Av mode] to [Auto] (p.173).
The shutter makes two shooting sounds during Live View shooting.
If you use flash, the shutter will make two sounds each time you
shoot (p.181).
During Live View and movie shooting, a white <
red <E> icon is displayed.
It indicates that the camera's internal temperature is high. If the
white < > icon displayed, the still photo's image quality may
deteriorate. If the red <E> icon is displayed, it indicates that the
Live View or movie shooting will soon stop automatically (p.201,
233).
Troubleshooting Guide
> or
361