I am going to be unable to explain why your server will not install SQL Express automatically. I recommend that you leave the SELECTServer application installed and do a manul ainstalltion of the SQL Express application. The SQLExpress application is developed by the Microsoft corporation. There is no "special "Bentley" build of the SQL Express application. The application delivered is the exact same version available for free from Microsoft. Please review the information in the link below.
Manual Installation of SQL Express (2008 - 2012)