Error: SQL server fails to install
This can usually be resolved by running the SQL installer outside of the 4Matrix Network Manager wizard.
Go to drive:\4Matrix\Files and you'll see SQL_2014.exe
Run it and make sure that you choose to use MIxed Authentication mode when asked. The install will take longer than if running via the 4Matrix installer, but it will provide a more detailed explanation if it fails again.
Once complete, go back to the 4Matrix install, go back a step and choose to use an existing server - choose the one you have just created manually.