This error message is displayed if the camera is set to auto-focus and the camera is unable to focus properly. A Canon DSLR will not take a photo if it is set to auto-focus and cannot focus properly and this will cause the photobooth shooting sequence to fail. There are two ways to avoid this problem:
1) Set the camera to manual focus by moving the AF/MF switch on the lens to the MF position or by setting the AF mode to “Remote Manual” in DSLR Remote Pro. Then pre-focus the lens to the correct subject distance by rotating the focus ring on the lens or by holding down the Ctrl key and using the mouse wheel if the AF mode is set to “Remote Manual” (requires DSLR Remote Pro for Windows v188.8.131.52 or later).
2) Increase the camera’s chances of being able to auto-focus by ensuring all focus points are active and there is sufficient light in the booth before taking the photo (cameras find it harder to auto-focus in poor light). Also avoid having a plain background otherwise the camera may not be able to auto-focus if people leave the booth before it has finished taking photos.
Method #1 guarantees the camera will take the photo but it does not guarantee that the subject will be in focus. If people move too close to the lens they may appear out of focus in the photos. Increasing the depth of field by using a smaller aperture (e.g. f/8 or f/11) should help.
Method #2 does not guarantee the camera will be able to focus and that it will be able to take a photo. The only way to guarantee that the camera will take a photo is to use manual focus.