Why can’t I turn on my camera during group FaceTimes, it only lets me do audio, I have a iPhone 6 Plus so it should let me?

Group FaceTime video calls require an iPhone 6s or later as per

https://support.apple.com/en-us/HT209022

We can guess, or you could tell us a whole lot more.

Here's one example.

The article you attached above starts with:

"FaceTime and FaceTime audio might not be available for all countries, regions, or carriers."

Have you checked this? Are you in a supported country? Region? Using a carrier that supports FaceTime?

And that's only the first bullet ...

Ok g

Ah, there it is! Thanks for discovering this. I looked for such a requirement, and failed to find it.