This error is located in the Agent log running a job with a SQL Server Job in the command entry.

MS SQL EXEC: SQL Server Error
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:

When setting up an SQL Server Definition, the Server Instance will need to be left blank if you are using the default instance of the SQL Server.