select em.* ,ct.Cities_NAME
 from Customer em , 
 Cities ct
 Where
 ct.Cities_ID = em.Customer_CITY -- it allows only matched id with city 
 and 
 Customer_GROUP is NULL  -- it accept when customer_group is null
 and 
 Customer_ENABLED not like '%FALSE%'  --it filter if customer enables is true
 and 
 Customer_TYPE not like '%User%'  -- it accepts customer type is not as user

不要在句子中使用逗号 FROM 条款。始终使用适当的、明确的、标准的 JOIN 语法：

select em.*, ct.Cities_NAME
from Customer em inner join
     Cities ct
     on ct.Cities_ID = em.Customer_CITY 
where em.Customer_GROUP is NULL and
      em.Customer_ENABLED and
      em.Customer_TYPE <> 'User';

出于某种原因，您列出了 ct 在查询中两次。您还应该限定查询中的所有列引用。