seconded to use a different group number. the error message is clear, and is consistent with the man pages error numbers returned from the linux groupadd command…that is, whatever OS image you are using (you don’t show a FROM line) has an existing group with the number 999, or somehow protects it.
If you’d like to find out what that group number is actually used for, you’d need to start a container using the same base image as for the sample you’ve shown, exec into the container (or start it with the interactive and tty flags), and check the contents of the group file. you could also try the command as written inside the container and see if the behavior is the same or different.