This error indicates that Runtime has attempted to connect to a MSSQL database, and was unable to do so due to a networking error.
Causes & Resolutions
This issue can be caused by a variety of things, such as:
TCP/IP connections are not enabled on the SQL Server. Enable this here: Start → Microsoft SQL Server → Configuration Tools → SQL Server Configuration Manager → SQL Server Network Configuration → Protocols for SQLExpress
No TCP loopback address defined.
Sometimes, TCP Dynamic ports can cause issues. To resolve this, remove All TCP Dynamic Ports and add a set value (by default, this is 1433) to all TCP Ports and restart your SQL Server Services > SQL Server
If none of the above have helped, it might be that the firewall is blocking traffic between Runtime and the SQL Server. To troubleshoot this, refer to your local system administrator.