This problem can be caused by many reasons, such as your iMessage and MMS Messaging hasn't been enabled, your cellular or Wi-Fi network is disconnected, or the video size is too large to transfer. To fix such a problem, you can go to Settings > Messages and toggle on the iMessage option. If the problem still exists, go to check whether there is an active internet connection on your device. If neither of the solutions is helpful, just take your device to the nearby Apple Store and ask for help.