string(66) "{"code":"SYSTEM_ERROR","message":"系统繁忙，请稍后重试"}"

这是什么错误，没有明显提示

public function get_serial_no(){
        $url = 'https://api.mch.weixin.qq.com/v3/certificates'
        $sign = $this->sign（$url，'POST'）
        $header[] = "User-Agent:"$_SERVER'HTTP_USER_AGENT'
        $header[] = 'Accept:application/json'
        $header[] = 'Authorization:WECHATPAY2-SHA256-RSA2048 '$sign
        $r = $this->doCurl$url''$header
        var_dump($r