Unknown SQL Exception 53 occurred. Additional error information from SQL Server is included below.
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: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)
There are a variety of reasons this may occur, most notable:
1) Not using a SQL Alias - this has been a problem will all SharePoint versions. If you attempt to use the SQL instance name it will work - sort of. If it is the default instance you might be OK but usually services like the User Profile will not. If you've not setup an alias before, the command is "cliconfg". Be sure to enable TCP/IP and Named Pipes (with TCP/IP as the priority).
2) An actual network problem - some have experienced this in networks where the DNS is flakey; often adding all of the farm servers to the Hosts file (on all servers) will correct.
3) Incorrect settings in SQL Server - if SQL is set to Named Pipes vs. TCP/IP, this will occur.
4) Something else - sometimes, simply re-running the installation will 'fix' the issue. I had this occur on a fresh system; I rebooted and ran again and the install completed.
Hope this helps!