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: 26 - Error Locating Server/Instance Specified)

The reason that you may get this error message is the client stack could not receive SSRP response UDP packet from SQL Browser. The following steps may help to isolate the issue:

1) Make sure your server name is correct.
2) Make sure your instance name is correct and there is actually such an instance on your target machine. [Some application converts \\ to \. If you are not sure about your application, please try both Server\Instance and Server\\Instance in your connection string]
3) Make sure the server machine is reachable, e.g, DNS can be resolved correctly; you are able to ping the server.
4) Make sure SQL Browser service is running on the server.
5) If firewall is enabled on the server, you need to put sqlbrowser.exe and/or UDP port 1434 into exception.