You need to replace the above with something like this:
<ConnectionString>Data Source=localhost\SQLEXPRESS; User Id=sa; Password=sambapos; Database=SAMBAPOS4;</ConnectionString>
Without knowing how you set up SQL Express, we cannot give better detail. Your Instance name (SQLEXPRESS) may be different, your User Id (sa) and Password (sambapos) may be different (or you may not need them at all if you are using Windows Authentication), and your Database name (SAMBAPOS4) may be different.