The error comes from your very first line when you're trying to create a char array called letter. Look at this example,
>> letters = ['a';'b';'c']
letters =
3×1 char array
'a'
'b'
'c'
now just add one more character to the last element,
>> letters = ['a';'b';'cc']
Dimensions of matrices being concatenated are not consistent.
do you see what's the problem? it's simply similar to numeric matrices. You cannot have one element more in just one row. In other words, the width of all the rows must be equal.
One alternative to this would be to declare that variable as a cell array,
>> letters = {'a';'b';'cc'}
letters =
3×1 cell array
'a'
'b'
'cc'
read about them by typing doc cell
