The default installation of SQLServer Express will create an Instance called computername\SQLEXPRESS.
Installing SQL Server LocalDB via SambaPOS installation creates an Instance called (localdb)\V11.0
Installing SQL Server LocalDB manually (outside of SambaPOS installations) will by default create an instance called (localdb)\MSSQLLocalDB
It is good to keep in mind that an Instance is not the same thing as a Database. For example, my SQLEXPRESS Instance contains 7 Databases with the names like [SambaPOS3], [SambaPOS4], [SambaPOS5]. Though there is nothing wrong with, and nothing stopping you from renaming the Instance during installation to be SAMBAPOS… just know that the Instance is not the Database.
The SQL Server LocalDB versions do not install as a Service; instead they are simply a Process called sqlservr.exe, which you should see in the Task Manager.
If you are using SQLExpress, the Instance is shown in Services. Ensure it is set to Start Automatically.
If the Service is not shown as Started, then you have an issue with Service Startup in Windows, likely due to the way it is set to start or due to permissions.
In your case, it looks like you need to set all terminals to attach to:
Data Source=Admin-PC\SAMBAPOS2; User Id=sa; Password=sambapos; Database=SambaPOS5
It is more reliable to give the server a Static IP and use that IP Address rather than the Computer Name.
Besides that, there is more to multi-terminal setups having to do with ensuring the SQL Browser Service is running on the Server, and allowing applications and/or ports through the Windows Firewall on the Server.
The link provided by @Jesse is a good resource.
I also went through a lot of trial and error, and came up with this: