First, you're loading the json wrong:
It should be
-> Array: Load from JSON string AJAX.LastData
Next, you're trying to get 1 dimensional data from a 2 dimensional array.
-> Text: Set text to Array.At(1,1)
That should be "B"
0,1 should be "b"
Have a question though, why are you using an array when you can uppercase("b") and get "B"
You could do a choose("a","b","c"... etc.) to get a random letter, stuff that into a variable and then compare user input to uppercase(variable)