why can’t i open a database file on a sqlite table?

At first, check the debug folder on your project that there it is

Next:

    var connectoinString=Path.Combine("Data source=", Environment.CurrentDirectory, "F4FDataSQLite_SingleTable.db")    
  //  or    
    var connectoinString=Path.Combine("Data source=", Application.StartupPath, "F4FDataSQLite_SingleTable.db")

using var con = new SQLiteConnection(connectoinString);
con.Open();