If you are 100% sure that connection string and all its parameters correct, check your Services and make sure SQL Server (SQLExpress) is running
If this is a new install, be sure that named pipes and tcp/ip connections are enabled in SQL Server Configuration Manager:
Also, set the ‘TCP Dynamic Ports’ to 0 and ‘TCP Port’ to 1433 for each interface:
If all the above are already configured, try and connect to the server using SQL Server Management Studio.
I dont get any issue right now when connecting to the server using SQL Server Management
i have made sure that the SQL Server is running but it still gives the error
i have also tried running the SQL Server using command prompt
Your instance name is MSSQLLocalDB not SQLEXPRESS. Are you using SQL LocalDB or do you have SQL Server Express installed?
If SQL Server Express is installed but the instance name is MSSQLLocalDB (for some reason) your data source would be Unish\MSSQLLocalDB
I have actually installed both on my pc
the following suggestion didnot work 
If there are both, then in SSMS you are connected to the localdb instance. Try and connect to the SQLEXPRESS instance.
Can you show me a screenshot of your running services (start->run->services.msc) where SQL Server is shown:
Right, so use Unish\MSSQLSERVER as the datasource in SSMS. If you can connect there you should be able to connect in SambaPOS.
Did you configure the instance to use sql server authentication (and not just windows authentication)?
Try these:
localhost\MSSQLSERVER
localhosti actually haven’t configured the instance to use sql server authentication
the localhost\MSSQLSERVER didnt work
but the localhost got connected to the SSMS
Try just ‘localhost’ as the data source in SambaPOS configuration
I am so grateful to Memo
you saved me so much time i had been trying to fix this for the past two days and was thinking about rebooting my whole pc
Thank You So Much
You’re most welcome. Have fun!!