Ok so I think you are confused… that does not install SQL Express 2012… that is installing LocalDB using SQL Express 2012… so you in fact do NOT have SQL Express installed. All you have is LocalDB.
So try this… go to your programs in control panel and uninstall LocalDB SQL Express. Completely uninstall it then run the SambaPOS installer again and choose that option. That might fix it.
So what I was referring too is a full SQL Express install and using a connection string in SambaPOS to connect to it. You would not choose either localdb install options for this.