I guess the issue is with the dot(.) in front of row.
Remove the dot and it should work fine.
If you have already checked Include column names checkbox in Read CSV activity, then you can directly use row("First Name").ToString
Also when you want to use the available functions of VB.net like ‘ToString’, try not to type them by yourself. Use the ‘intellisense’ feature. Example, in your case, after row(“First Name”) if you keep a dot and type a few letters of the function you want to use, you should be getting the available functions that you can use for the data type. Choose from one of them so that you don’t force the tool to do something it cannot.