That is a frustrating one, but it happens more often than you’d think! Usually, when the flash control menu is greyed out, it’s because the camera doesn't detect a compatible flash sitting in the hot shoe, or the flash itself isn't powered on and ready to communicate with the body.
The first thing I’d check is the seating - sometimes just sliding the flash out and back in to ensure the pins are making a clean connection does the trick. Also, double-check that you aren't in a completely silent shutter mode or a high-speed continuous release mode, as those settings will often disable flash functionality automatically to keep up with the frame rate.
