Thank you for using Apple Support Communities! I understand that you only have the option to make a Group FaceTime audio and not video call on your iPhone 6. This behavior is to be expected as the iPhone 6 does not currently support making Group FaceTime video calls.
Learn more about using Group FaceTime in the following resource:
Use Group FaceTime on your iPhone, iPad, and iPod touch - Apple Support
Specifically, check out this section for more information about devices that can be used for Group FaceTime video calls:
To use Group FaceTime video calls, you need an iPhone 6s or later, iPad Pro or later, iPad Air 2, or iPad Mini 4 with iOS 12.1. Earlier models of iPhone, iPad, and iPod touch that support iOS 12.1 can join Group FaceTime calls as audio participants.
FaceTime isn't available in all countries or regions.
Take care!