Canon EOS REBEL T8i Advanced User's Manual page 717

The external Speedlite does not fire.
Make sure the external Speedlite is securely attached to the camera.
The Speedlite always fires at full output.
If you use a flash unit other than an EL/EX series Speedlite, the flash will always fire at
full output ( ).
The flash always fires at full output when [Flash metering mode] in external flash
Custom Function settings is set to [TTL flash metering] (autoflash) (
Flash exposure compensation cannot be set for the external Speedlite.
If flash exposure compensation is set with the external Speedlite, compensation amount
cannot be set with the camera. When the external Speedlite's flash exposure
compensation is canceled (set to 0), flash exposure compensation can be set with the
camera.
Remote control shooting is not possible.
When taking still photos, set the drive mode to <
movies, set [
Check the position of the remote control's release timing switch.
If you are using Wireless Remote Control BR-E1, see
To use a remote control for time-lapse movie recording, see
I cannot shoot with Live View.
Set [ : Live View shoot.] to [Enable].
The shutter makes two release sounds during Live View shooting.
If you use flash with Live View shooting, the shutter will make two release sounds each
time you shoot.
The electronic level is not displayed in Live View shooting.
The electronic level is not displayed in Live View shooting when the AF method is set to
[ +Tracking].
: Remote control] to [Enable] (
> or <
).
Wireless Remote Control
717
).
> ( ). When recording
Time-Lapse Movies.
BR-E1.