ERROR 1091 (42000): Can't DROP 'created_date'; check that column/key exists
If you want to drop the index that you created previously, the correct statement would be: 

alter table bill_item drop index idx_comp;

created_date is the name of one of the columns in your table bill_item.

Note: To find the index name use 
SHOW INDEXES FROM &lt;table&gt; if you used a different naming convention than prepending 'idx_'