Why can’t I enable call forwarding on my iPhone XR ?

According to this link:

Set up call forwarding and call waiting on iPhone - Apple Support

your carrier needs to have a GSM network.

I followed the steps in the instructions and discovered that my phone doesn't show Call Forwarding as an option in my phone settings either.

That's ok with me because whenever I need to forward my phone, I enter *72[the phone number] then press the call button.

Example: *725551234567

Then to cancel call forwarding, I enter *720 and press the call button. I just noticed that on their website, Verizon is saying *73 to cancel the call forwarding. I can't keep up with them, there was a while they said to do *720 and it's the way that I'm still doing it today.

My service is through Verizon. You would need to double check to see how call forwarding is activated through your carrier.

What happens when you try?

If you are on a legacy CDMA carrier such as Verizon, there are no settings on the phone itself for enabling call forwarding. You'll need to directly dial in the codes.

Here are the Verizon instructions:

https://www.verizon.com/support/knowledge-base-181139/

Other carriers may be the same or similar.