Hello, I understand how frustrating this can be. You can easily create a conference call using the phone app. I am including a link to a guide to assist you with setting up a conference call on your Galaxy S21 https://www.samsung.com/au/support/mobile-devices/setup-conference-call/.
it is 'by default' but I have come across cases that default stuff were not actually enabled and the carriers had to enable it manually or reboot the service to the device.
that being said, who is your carrier? share the phone's model number & software version.
You can reproduce the issue and reporting to samsung for help.
Samsung members app > get help > error report
I contacted Fi and reported the issue to both Google Fi and Samsung already. I will contact Google again to see if they can enable it manually later as you said. Thank you 😌
Issue persists 😭