Google Meet originally had poor controls for moderating when students can join Meets. Unfortunately, unmoderated Meets have sometimes witnessed inappropriate student behavior. If students have been active in a Google Meet without teacher supervision, that Meet is added to the block list. You will need to create a new Meet.
Google has significantly upgraded Meet moderation controls in recent weeks. These improvements only apply to Meets created since the first of October. Create a new Meet in Google Classroom or use a nicknamed Meet with students. Classroom and nicknamed Meets have the latest moderation capabilities and can prevent students from joining Meets without adult supervision. A simple best practices guide can be found here.