TCP/IP is not enabled on the SQL server.
TCP/IP network traffic needs to be enabled on the SQL server, so that remote connections will be allowed on the SQL server.
- Run the SQL Server Configuration Manager.
- Click on the SQL Server Network Configuration.
- Select Protocols for SQLEXPRESS and check if the TCP/IP Protocol is enabled.
- If the TCP/IP is disabled, double-click on it and change the Enabled row status to Yes.
- In order to finish the adjustment, select SQL Server Services, click on SQL Server and restart it.
- SQL connection should now be available.