Flash could be jammed. Press the flash up button and carefully try to pop it up by putting a finger nail in the slot between flash and body. Don't force it.
The error means the flash is "obstructed" and wouldn't pop up when the camera tried to relase it.
The flash is spring-loaded but held down by a tiny hook. When you press the pop-up flash button OR if the camera tries to pop-up the flash automatically it just has a simple solinoid that release the hook and a spring is suppose to make the flash pop up.
If there's anything jamming the the hinge (a bit of sand) or perhaps some gunk... it'll stay down. The flash has a tiny micro-switch (like the button on the refrigerator door that knows to turn the light on when it's opened) so the camera knows if the flash actually did pop up.
Push the button to tell the flash to pop up (you'll hear the buzz of the solenoid release the hook) and at that point you should be able to GENTLY lift up the flash.
You may need to inspect and clean the hinge. DO NOT USE OIL. Oil will only attract dust. If it's not clean, use denatured alcohol to clean away any gunk (because it'll self-evaporate and not leave any residue behind.) Just a drop or two should do it.
Depending on the mode you are in the flash may not pop up automatically. Try switching modes going from Green Box to Program, Av, Tv to M attempting to raise the flash manually.
If the issues persist, you may need to consider setting up a repair for your product on our web site.
Most likely the flash linkage is dirty. You can help the flash pop up with a finger nail but you must do it when the T5i is trying to pop it up. You must be quick as the T5i gives up quickly. Then you can't pry it up.
Once you succeed clean the linkage with alcohol and a q-tip.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.