My Cannon Rebel T3i won't take pictures in the dark with the flash on.

[vivmar18]

Does anyone know how to fix this issue, please?  Thank you so much!

[kvbarkley]

It is probably not achieving focus. Have you tried it in manual focus?

Do you have it set for "disco flash"?  i.e., use the pop-up flash for focus assist.

[TCampbell]

This is likely because there isn't enough light to focus. 

If the camera is in "One Shot" mode (which is the default) then it will not take a shot until it can confirm that it has locked focus.  It will take a shot if you switch to manual focus.  It will also take a shot if it is in "AI Servo" focus mode (however that mode will take a shot even if it hasn't focused... use with caution or you may be find your shots weren't actually focused.)

The camera has auto-focus assist that can use pulses of light from the flash to attempt to provide enough light to help the camera focus.  But this function can be disabled via in-camera menus (it is enabled by default).  This feature will only be used if the built-in flash is raised up (Note: If you are in P, Av, Tv, M or A-Dep mode it will not flip up on it's own.  There is a button on the front of the camera - left side - just above the lens release which resembles a lightning bolt which can be used to manually pop-up the flash.)

Some Canon external speedlites (such as the Speedlite 600EX-RT and 600EX II-RT, the 430EX III-RT and 430EX II) have a much better AF-assist beam that uses a red pattern (the light is behind the red lens in the lower part of the speedlite body) and the camera can use that pattern to lock auto-focus even in complete darkness.)