Hi!
First of all update your iPhone to the latest iOS version (currently iPhone 6 can be updated to iOS 12.4.4).
If it won't help, try going trough this:
https://support.apple.com/en-us/HT201422
If there is still no luck, you should do the test. Try putting the SIM card, you're trying to activate FaceTime with to another iPhone and check if it'll work. Meanwhile, ask for someone's SIM card who has his FaceTime working for him. This way you'd be able to determine if it's a carrier's or SIM card's fault.
If after this test, you'd find that FaceTime works on your phone with a different SIM, you should contact your carrier and ask what's going on.
Lastly, if you'd find out that your phone's FaceTime isn't working with ANY SIM card, even it FaceTime works trough this SIM on other iPhone - try do the factory reset (https://support.apple.com/en-us/HT201252), set it up as a new phone and try activating the FaceTime with the SIM that worked on another phone right after you get to the home screen. It it'll fail, contact Apple support.