This error is caused when no connection to your Linnworks database can be established. The usual cause for this is due to a firewall that is restricting access to SQL. This could be both a software firewall such as Windows Firewall or McAfee, or a firewall built into your network router.
Disable each firewall application in turn. If this works, then re-enable the firewall and allow Linnworks Order Management and SQL through the firewall.
Disable your routers firewall. If this works then re-enable and try allowing port forwarding on Port 1433.