Coinpayments create_transaction "ERROR: Invalid command!"

PHP Snippet 1:

 function curl_coin_payment ($postdata = array()) {
        $url = "https://www.coinpayments.net/api.php";
        
        $payload = $postdata;
        $payload['key'] = 'public_key';
        $payload['version'] = 1;
        $payload = http_build_query($payload, '', '&');
        $api_secret = 'private_key';
        $apiseal = hash_hmac('sha512', ($payload), $api_secret);
        
        $ch = curl_init();   curl_setopt($ch, CURLOPT_URL, $url);        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_POSTFIELDS, ($payload));   curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        $headers =   [
            "HMAC: $apiseal",
            "Content-Type:  application/x-www-form-urlencoded",
        ];                 
        curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);     $request = curl_exec ($ch);     curl_close ($ch);
        return $request;
}