My Group FaceTime doesn’t work

All the users need to be on iOS 12.1, and group video requires an iPhone 6s or above. See other information here, Use Group FaceTime on your iPhone, iPad, and iPod touch - Apple Support

You told us nothing about your iPhone equipment.

Video Group FaceTime, for example, requires an iPhone 6S or later. Do you meet this requirement?

https://help.apple.com/iphone/12/#/iph405ab67de