If there is no data between <ConnectionString > and </ConnectionString> SambaPOS doesn’t what to connect to.
This seems to be an issue with your installation on more than one occasion.
I recommend you reinstall SambaPOS. Select only what is shown:
Then install SQL Server Express 2017. Login to SambaPOS with the default Admin password of 1234 and follow the directions for configuration in the first post of the thread SambaPOS Database Configuration under " SQL Server Configuration".