Expected Primary Expression error on signature function

Question

With this isValidSignature function:

// NOTE: This method not safe, contract may attack by signature replay.
    /**
     * Verify if the signer has a valid signature as claimed
     * @param signer The address of user who wants to perform an action
     * @param signature The signature signed by the signer
     */
    function isValidSignature(address signer, bytes memory signature)
        public
        view
        returns (bool)
    {
       
        bytes32 messageHash = keccack256(abi.encodePacked(***byte***(0x19), byte(0), address(this), signer));
        return ECDSA.recover(messageHash, signature) == signer;
    }

I get linting for byte(0x19) stating there is an "Expected Primary Expression". Why does this happen?