Why does Vesta always give an error?

Question

PHP

Good afternoon
There is a server with vesta on DigitalOcean. I
use the example from the vesta api documentation to create subdomains

$postvars = array(
            'user' => $this->vst_username,
            'password' => $this->vst_password,
            'returncode' => $this->vst_returncode,
            'cmd' => 'v-add-domain',
            'arg1' => $this->username,
            'arg2' => $this->session->adding_domain // domain
        );
        $postdata = http_build_query($postvars);
        $curl = curl_init();
        curl_setopt($curl, CURLOPT_URL, 'https://' . $this->vst_hostname . ':8083/api/');
        curl_setopt($curl, CURLOPT_RETURNTRANSFER,true);
        curl_setopt($curl, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($curl, CURLOPT_SSL_VERIFYHOST, false);
        curl_setopt($curl, CURLOPT_POST, true);
        curl_setopt($curl, CURLOPT_POSTFIELDS, $postdata);
        $answer = curl_exec($curl);
        if($answer == 0) {
            echo "Domain has been successfuly created\n";
        } else {
            echo "Query returned error code: " .$answer. "\n";
        }

Always returns error code 4 (Object already exists)
And - in the absence of a domain - it creates it and says that the domain already exists ....
where is the error?
I'm writing on CodeIgniter if it helps the solution...