When importing an existing project that you have not setup you may have when executing a SqlConnection this kind of error : flag is not supported.
Error Message: The user instance login flag is not supported on this version of SQL Server.
This can occur when the previous developer had in his connection string a reference to “User Instance“. This need to be removed.
A user instance is like a normal database instance but it is created on demand. Normally, instances are created during the creation of the database.
To remove this error, you just need to remove this configuration of the Sql connection string.