Cannot convert a nonclustered index to a clustered index online using DROP_EXISTING option.

Error Message:
Msg 1968, Level 16, State 1, Line 2
Cannot convert a nonclustered index to a clustered index online using DROP_EXISTING option.

Severity level:
16.

Description:
This error message appears when you try to convert a nonclustered index to a clustered index by using the DROP_EXISTING option.

Consequences:
The T-SQL statement can be parsed, but causes the error at runtime.

Resolution:
Error of the Severity level 16 are generated by the user and are corrigible by the user. The statement cannot be run this way. When you want to change a nonclustered index to a clustered index you can not use the DROP_EXISTING option. You have to use a DROP INDEX & and CREATE INDEX statement combination instead.

Versions:
This error message was introduced with SQL Server 2005.

Example(s):
IF OBJECT_ID('dbo.t') IS NOT NULL
    DROP TABLE dbo.t;
GO

CREATE TABLE dbo.t
(
    c1 INT
    CONSTRAINT CIX_t_C1 PRIMARY KEY NONCLUSTERED
)
GO

CREATE UNIQUE CLUSTERED INDEX CIX_t_C1 ON dbo.t(c1)
    WITH (
    DROP_EXISTING = ON,
    ONLINE = ON   
    );
   
DROP TABLE dbo.t;

Remarks:
In the above example we try to convert a nonclustered index to a clustered index by using the DROP_EXISTING option. This raises the error.