Problem:

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)

Solution:

If you are using named instance on a port other than the default, make sure SQL Browser is running, or configure the custom port in the SQL Estate Manager UI.