My back up guidelines do not appear when backing?

Did this just start happening? Has any work been done recently? Do you have anything plugged into the trailer tow connector? There are some settings you can access through the steering control buttons, some involve the rear-view camera, your owners manual covers how to access them as they are slightly different depending on the actual options installed.
If none of those help, then it may be necessary to use a scan tool to access the system and see if there is a software update or refresh to enable them.