Unable to activate or deactivate call forwarding on iPhone

Sorry, but you're going to have to work this out with your carrier. Call forwarding is a carrier feature, not a function of the phone.

How about calling your carrier, as they provide the call forwarding feature.

As you say you are unable to activate or deactivate, how do you know? Is it currently activated and you can’t deactivate it, or do you want to activate it and can’t?

You are still rather vague, and didn’t answer everything. Who is your carrier? If it’s Verizon, follow Smilin-Brian’s guidance.

For any other carrier, first follow that same advice, and, if you don’t see Call Forwarding go to Settings/Apps/Phone. You should see an entry for Call Forwarding. Is it there? If it is, tap on it. Do you see a switch? Can you turn it on and off? What position is it in?

The following is from a post I saw yesterday:

If it’s off, turn it on and enter a valid phone number.

If it is on, turn it off, then repeat the above instructions

If neither works call your carrier ask to speak to a senior tech, and ask them to reprovision your account, then turn it on, enter a valid number, then turn it off.

ADDENDUM: It is always a bad idea to control your account with vertical service codes, unless specifically you are told to by your carrier or their website. The codes are not universal, they are carrier specific, and entering the wrong code can break your cellular service. Here are some examples: https://en.wikipedia.org/wiki/Vertical_service_code

Try the following:

1. Open Settings
2. Tap Cellular
3. Tap Carrier Services

⇣ Example: ⇣

Have a nice day! 👋🏼😉