Canon EOS REBEL T8i Advanced User's Manual page 714

Shooting-related problems
The lens cannot be attached.
The camera cannot be used with RF or EF-M lenses (
The viewfinder is dark.
Install a charged battery in the camera (
No images can be shot or recorded.
Make sure the card is properly inserted (
Slide the card's write-protect switch to the Write/Erase setting (
If the card is full, replace the card or delete unnecessary images to make space
( , ).
Shooting is not possible during One-Shot AF when the focus indicator <
viewfinder is blinking, or when the AF point is orange during Live View shooting or movie
recording. Press the shutter button halfway again to refocus automatically, or focus
manually ( ,
The card cannot be used.
If a card error message is displayed, see Removal.
An error message is displayed when the card is inserted in another
camera.
Since SDXC cards are formatted in exFAT, if you format a card with this camera and
then insert it into another camera, an error may be displayed and it may not be possible
to use the card.
I have to press the shutter button twice to take a picture.
Set [10: Mirror lockup] in [ : Custom Functions(C.Fn)] to [0:Disable] (
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