T-SQL: Cannot drop the index because it does not exist or you do not have permission.

Quick tip:   The table name and index name are required to drop an index, otherwise SQL Server will return an error.

e.g.

DROP INDEX [dbo].[IX_MyIndex]

fails with …

Msg 3701, Level 11, State 6, Line XX
Cannot drop the index 'dbo.IX_MyIndex', 
because it does not exist or you do not have permission.

However,

DROP INDEX [dbo].[MyTable].[IX_MyIndex]

will work fine if it exists and you have sufficient permissions. (Use Select user_name() to determine your user context, which will help to verify the permissions being used.)