Constraint name '%.*ls' not permitted. Constraint names cannot begin with a number sign (#).

Error Message:
Msg 8166, Level 16, State 0, Line 2
Constraint name '%.*ls' not permitted. Constraint names cannot begin with a number sign (#).

Severity level:
16.

Description:
This error message appears when you try to create a constraint beginning with the # number sign.

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. You can not create a constraint beginning with the # number sign. You must use another name.

Versions:
All versions of SQL Server.

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

CREATE TABLE dbo.t
(
    c1 int NOT NULL
    CONSTRAINT #PK_t PRIMARY KEY
);

Remarks:
In the above example we try to create a PRIMARY KEY constraint with a name of #PK_t. This raises the error.