_beyousid_’s screenshot showed the active call indicator. Because the microphone is being used by the call, it cannot also be used to record the audio track for a video recording. Hang up your call and try recording again. If you are not in a Cellular, FaceTime Audio, or FaceTime Video call, are not using a 3rd party app for any type of call, and have no application using your microphone in the background (a call will show a green bar at the top of your screen on a Touch ID iPhone, and a green Bible around the time stamp on a FACE ID iPhone) you’ll need to turn your iPhone on and off by holding the Sleep/Wake button, swiping the onscreen slider, then holding the Sleep/Wake button again to power your iPhone back on. If the Camera app stop will not record video, force restart it. The method by which you force restart an iPhone varies by model, so, if you don’t know how to force restart your iPhone, search for “force restart [your iPhone model]. If you don’t know what model iPhone you have, go to Settings>General>About and you will see your iPhone’s model name among other listings; the model name is what you’ll need in your search. If you still need help with a force restart, chat with Apple support or call at 1(800) MY-APPLE (1(800) 692–7753).
Big picture: if you have something, app, call, etc, using your microphone, video won’t work. Close all apps that may be using your microphone, power cycle, and force restart your iPhone, if necessary. Contact Apple via chat or call at 1(800) MY-APPLE (1(800) 692–7753) if you need help with any of those steps or if they don’t work. Try using your Camera App’s video function and see if it works, now.