[SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 52 - Unable to locate a Local Database Runtime installation. Verify that SQL Server Express is properly installed and that the Local Database Runtime feature is enabled.)]

The error message has stated all. SQL Database inside App_Data is for developing/testing purpose only.

You need a real SQL Server when your application goes to live. So you need to install SQL Server either in the same server that the application is hosted or a dedicated SQL server. Then attach that database.

[SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 52 - Unable to locate a Local Database Runtime installation. Verify that SQL Server Express is properly installed and that the Local Database Runtime feature is enabled.)]

LocalDB (SQL Database inside App_Data) is primarily intended for developing and testing purpose only. When you publish to production, it will not be copied with your source code.

You need a real SQL Server when your application goes to live. So you need to install SQL Server either in the same server that the application is hosted or a dedicated SQL server. Then attach that database.