Commands out of sync; you can't run this command now PHP/MySQL 5.6

Question

I am trying to execute following query using mysqli (PHP).

        public function brandAvailabilities($start_date, $return_date)
        {
            $query ="
            SELECT brand_id, label, MIN(free) 
                FROM
                (
                    SELECT B1.brand_id, CONCAT(B1.brand,' ',B1.model) as label, B1.stock - COUNT(*) as free
                        FROM 
                            VIEW_VEHICLE_BOOKINGS B1
                            JOIN
                            VIEW_VEHICLE_BOOKINGS B2
                            ON  B2.brand_id = B1.brand_id AND
                                B2.start_date <= B1.start_date AND
                                B2.return_date > B1.start_date
                        WHERE B1.start_date <= ? and B1.return_date >= ?
                        GROUP BY B1.brand_id, B1.start_date
                    UNION
                    SELECT DISTINCT B3.id, CONCAT(B3.brand,' ',B3.model) as label, B3.stock as free
                        FROM VEHICLE_BRANDS B3
                ) T GROUP BY brand_id;";

            $paramType = "ss";
            $paramArray = array($return_date, $start_date);
            return $this->db->select($query, $paramType, $paramArray);
        }

VIEW_VEHICLE_BOOKINGS is view created as (not using above query, It is done already before)..

CREATE OR REPLACE VIEW VIEW_VEHICLE_BOOKINGS
AS
SELECT B.id, B.start_date, B.return_date, C.brand, C.model, C.id as brand_id, C.stock
FROM 
    BOOKING_ASSIGNMENTS A
    JOIN
    VEHICLE_BOOKINGS B
    ON A.booking = B.id
    JOIN
    VEHICLE_BRANDS C
    ON A.vehicle = C.id

db->Select function is defined as

        public function select($query, $paramType="", $paramArray=array())
        {
            if(!($stmt = $this->conn->prepare($query)))
            {
                echo "<br/>ERROR : Prepare failed : (" . $this->conn->errno . ") " . $this->conn->error;
                echo "<br/>ERROR for QUERY '".$query."' PARAMS '".$paramType."'";
            }

            if(!empty($paramType) && !empty($paramArray))
            {
                $this->bindQueryParams($stmt, $paramType, $paramArray);
            }
            
            $stmt->execute();
            $result = $stmt->get_result();
            if($result)
            {
                if ($result->num_rows > 0) 
                {
                    while ($row = $result->fetch_assoc()) 
                    {
                        $resultset[] = $row;
                    }
                }
                else 
                {
                    echo "<br> No Record Found to display";
                }
            }
            else 
            {
                echo "<br> Database error ** ".mysqli_error($this->conn)." **";
            }

            if (! empty($resultset)) 
            {
                return $resultset;
            }
        }
    

Saying error

Commands out of sync; you can't run this command now

I am have no idea who to fix it. Help to fix it. Thanks in advance

PHP 7.0
MySQL 5.6