You should use the 's' option in input() to avoid interpreting the input, e.g., input A will be stored as the letter A rather than MATLAB looking for a variable called A.
grade = input('Enter your letter grade: ','s');
And you will also need quotes around the letters in all your case statements, e.g.:
switch grade case {'A','a'} % etc.
Or you could switch on upper(grade) or lower(grade) to only have to include one letter case in each case statement:
switch lower(grade) case 'a' % etc.