Go into betaflight. Go to OSD tab. Change to pal or ntsc or the opposite of what it’s on or auto. Save and reboot.
If your OSD is configured to a specific standard, PAL or NTSC and your camera is the opposite, OSD will not display in your video. You can either tell the flight controller what standard it is or if your camera supports changing its video format, you can change it from within the camera.
Still Missing OSD
You could have wired the onboard OSD wrong. You can check out more about onboard flight controller OSD wiring here.