letter = string.ToUpper(letter);

I am guessing that letter is defined as a char in which case you need to use char.ToUpper(letter).

In the future when something isn't working and you're seeking help it's always helpful to provide the exact error message you are gettingg and when you are getting it, e.g. at runtime or compile time. It helps you get help faster. Thanks.

Hope this helps.

The String class does not have a static method named ToUpper() that you can call in that way, instead you should call the ToUpper() method on the string instance you wish to carry it out on ala:

letter = letter.ToUpper();