While it comes pre-installed on some Razer hardware (like the Kraken Ultimate or BlackShark V2), many Razer headsets require an activation code to unlock the full suite of features in Razer Synapse.
If your headset didn't come with THX built-in but is a supported Razer product (like the Barracuda X), you can earn an activation code or a discount by registering it: thx spatial audio free activation code