使用聚合函数 Sum() 没有 Group By 在这里没有意义。你需要做一个 Group By 在 s.system_number .
然后，你可以使用 Having 条款来考虑 difference_points 小于0。
请尝试以下操作：

SELECT 
    s.system_number,
    SUM(s.total_points),
    SUM(p.licensed_points),
    SUM(p.licensed_points) - SUM(s.total_points) AS difference_points
FROM 
    server AS s 
    LEFT JOIN 
        practice AS p ON p.system_number = s.system_number 
GROUP BY 
  s.system_number 
HAVING 
    difference_points < 0