MySQL - This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery

Instead of using IN, you can use JOIN

SELECT v.VID, v.thumb
FROM video AS v
INNER JOIN
     (SELECT VID
     FROM video
     WHERE title LIKE "%'.$Channel['name'].'%"
     ORDER BY viewtime DESC
     LIMIT 5) as v2
  ON v.VID = v2.VID
ORDER BY RAND()
LIMIT 1

You can use below to bypass this error.

$Last_Video = $db->fetch_all('
    SELECT VID, thumb
    FROM video
    WHERE VID IN (select * from (
        SELECT VID
        FROM video
        WHERE title LIKE "%'.$Channel['name'].'%"
        ORDER BY viewtime DESC
        LIMIT 5) temp_tab)
    ORDER BY RAND()
    LIMIT 1
');

You don't need a subquery here. Try this:

 SELECT VID, thumb
 FROM video
 WHERE title LIKE "%'.$Channel['name'].'%"
 ORDER BY RAND() DESC
 LIMIT 1

In MySQL 5.0.26 and later, you will get an error:

MySQL does not support LIMIT in subqueries for certain subquery operators:

Reference.