Encourage your students to keep their video turned on. If you want to require it, you must provide a no-penalty alternative for those who cannot turn their video on. You cannot impose a penalty on those who don't comply. It's important to remember the reasons some participants keep their video off. Some participants may have privacy issues and/or may have personal reasons for not turning on their video. Some computers don’t have sufficient bandwidth to manage a meeting in which everyone’s video is on, especially if several people in a household are sharing a single internet connection. Moreover, a video requirement can create a great deal of stress for some students.
If you do require "video on," you should provide in writing a clear pedagogical rationale (on your syllabus or other course document) why video presence is essential in your class (or during certain parts of your class). Because there still may be students who cannot turn their video on, you should also develop alternative ways of assessing the extent to which students are engaged and participating. For instance, you might require them to submit questions and comments in the “chat” or orally, or complete an assignment that asks them to reflect on some aspect of the synchronous class session, etc. The important thing is to be very clear about what your video requirements are, to provide pedagogical justification for those requirements, and to provide an alternative for students who cannot comply.