If the player spells a ten or more letter word, they should receive a 200 point bonus

Find the GetScoreForWord method. The for loop will calculate the score for the word played, the if statement adds any bonus points.

private static int GetScoreForWord(string Word, Dictionary<char, int> TileDictionary)
        {
            int Score = 0;
            for (int Count = 0; Count < Word.Length; Count++)
            {
                Score = Score + TileDictionary[Word[Count]];
            }
            if (Word.Length > 7)
            {
                Score = Score + 20;
            }
            else if (Word.Length > 5)
            {
                Score = Score + 5;
            }
            return Score;
        }

Change the if statement by adding a new condition, and change the current >7 condition to an else if instead. See below:

if (Word.Length > 10)
{
  score = score + 200;
}
else if (Word.Length > 7)
{
  Score = Score + 20;
}
else if (Word.Length > 5)
{
  Score = Score + 5;
}