The database principal owns a schema in the database, and cannot be dropped. (Microsoft SQL Server, Error: 15138)

USE [master]
GO
CREATE LOGIN [TESTUSER] WITH PASSWORD=N'Password123_4U', DEFAULT_DATABASE=[TESTDB], CHECK_EXPIRATION=OFF, CHECK_POLICY=OFF
GO
use [master];
GO
USE [TESTDB]
GO
CREATE USER [TESTUSER] FOR LOGIN [TESTUSER]
GO
USE [TESTDB]
GO
ALTER ROLE [db_datareader] ADD MEMBER [TESTUSER]
GO
USE [TESTDB]
GO
ALTER ROLE [db_datawriter] ADD MEMBER [TESTUSER]
GO
/*------------------------
USE [master]
GO
CREATE LOGIN [TESTUSER] WITH PASSWORD=N'Password123_4U', DEFAULT_DATABASE=[TESTDB], CHECK_EXPIRATION=OFF, CHECK_POLICY=OFF
GO
use [master];
GO
USE [TESTDB]
GO
CREATE USER [TESTUSER] FOR LOGIN [TESTUSER]
GO
USE [TESTDB]
GO
ALTER ROLE [db_datareader] ADD MEMBER [TESTUSER]
GO
USE [TESTDB]
GO
ALTER ROLE [db_datawriter] ADD MEMBER [TESTUSER]
GO
------------------------*/
Completion time: 2024-12-02T11:22:44.0652732+05:30
--Create the user
USE [TESTDB]
GO
CREATE USER [TESTUSER] FOR LOGIN [TESTUSER] WITH DEFAULT_SCHEMA=[dbo]
GO
ALTER ROLE [db_datareader] ADD MEMBER [TESTUSER]
GO
ALTER ROLE [db_datawriter] ADD MEMBER [TESTUSER]
GO
USE [TESTDB]
GO
ALTER AUTHORIZATION ON SCHEMA::[db_datareader] TO [TESTUSER]
GO
USE [TESTDB]
GO
ALTER AUTHORIZATION ON SCHEMA::[db_datawriter] TO [TESTUSER]
GO
DROP USER [TESTUSER]
GO
/*------------------------
DROP USER [TESTUSER]
GO
------------------------*/
Msg 15138, Level 16, State 1, Line 32
The database principal owns a schema in the database, and cannot be dropped.
Completion time: 2024-12-02T11:28:28.1249217+05:30


ALTER AUTHORIZATION ON SCHEMA::[db_datareader] TO [dbo]
ALTER AUTHORIZATION ON SCHEMA::[db_datawriter] TO [dbo]
GO

/*------------------------
ALTER AUTHORIZATION ON SCHEMA::[db_datareader] TO [dbo]
ALTER AUTHORIZATION ON SCHEMA::[db_datawriter] TO [dbo]
GO
------------------------*/
Completion time: 2024-12-02T11:29:59.8570656+05:30

DROP USER [TESTUSER]
GO
/*------------------------
DROP USER [TESTUSER]
GO
------------------------*/
Completion time: 2024-12-02T11:30:23.8254282+05:30