Try it a few times, it threw that error several times for two iPhones here immediately after updating but after the third go it activated and working away fine on both now.
After updating I had the same issue. I fixed it by doing this.
Firstly, to get the WiFi calling menu to even appear i had to restart my phone twice.
Secondly, for the webpage to open I had to turn off Wifi and use mobile data then restart my phone and only attempt to toggle it on while on mobile data. Once the webpage would show up it would be blank and not load. I solved this by Disabling any encrypted dns provider I had set (Settings -> General -> VPN, DNS and Device Management -> DNS -> Set to Automatic) After I did that the website did load with the terms of service and I tried accepting and it would throw an error saying to try later. Eventually after I restarted my phone it allowed me to accept it and wifi calling started to work. Try doing all of this while on mobile data and it should start working for you