Focusing in low light can be difficult for the autofocus system and sometimes you will need to use the manual focus, you can select the magnified image and see if that helps
See the online manual - page98 refers to how to magnify the image - using the magnify button - changes between 5X 10X and full screen
To help the camera autofocus there is an assist beam - this takes the form of flashes from flash to help the AF system focus - make sure it is ON - there is a custom function full details on Page 157
C.Fn III: Auto focus/Drive
6 AF-assist beam firing
FYIThe support site for this camera is here, including online manual
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