The error
The database principal owns a schema in the database, and cannot be dropped.
Query to identify users:
SELECT s.name
FROM sys.schemas s
WHERE s.principal_id = USER_ID('user');
Query Executed
ALTER AUTHORIZATION ON SCHEMA::db_owner TO dbo;
ALTER AUTHORIZATION ON SCHEMA::db_datareader TO dbo;
ALTER AUTHORIZATION ON SCHEMA::db_datawriter TO dbo;
finally
drop user usertobeDeleted
based https://stackoverflow.com/questions/35434389/how-to-drop-a-sql-server-user-with-db-owner-privilege
