How can I set my streamwriter?

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C#

WPF

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Hello, I would like to set my StreamWriter to write my desired value in the line (time in this case) and then sort by value as it is in the leaderboard. I'm trying to create a game in the WPF Application. And my StreamWriter is one of the methods. The problem is that I get an error: System.UnauthorizedAccessException: 'Access to the path 'C:\Ranking.txt' is denied.' So I don't even check if I used my sorting correctly.

What I have tried:

readonly string path = @"C:\Users\info\Desktop\ranking.txt";
private void writeText(string strPath, TimeSpan tsDuration)
        {                   
            StreamWriter str = new StreamWriter(strPath, true);
            string[] line = File.ReadAllLines(strPath);
            Func<string, int> sortBy = line => int.Parse(line.Substring(0, 13));
            var sortedLine = line.Skip(1)
                                  .Take(line.Length - 2)
                                  .OrderBy(sortBy)
                                  .ToArray();
            
            str.WriteLine(tsDuration);
            str.Close();
        }

I know there can be four possibilities why this is so:
1. The caller does not have the required permission.
2. The file is an executable file that is in use.
3. Path is a directory.
4. Path specified a read-only file.
But I don't know how to solve the problem.

Thank you for advice.