FtpWebRequest returns error 550 File unavailable

This error can be caused because of several reasons like file is not present on server, security permissions on file etc. etc.

First you need to find out the exact cause of error.
This can be achieved by using following code-

try
{
        //Your code
}
catch(WebException e)
{
        String status = ((FtpWebResponse)e.Response).StatusDescription;
}

Once you get the exact cause of error, you can go forward to solve it.

Here are some links you can refer