Your code has several features that need to be fixed, such as your using %d format specifier with a character vector, use of the loop iterator name within the format string, getting rid of pointless duplication of data, and moving the main folder creation out of the loop. Probably you want something like this:
D = 'C:\Users\sheri\Desktop';
F = 'Categories';
if ~exist(fullfile(D,F),'dir')
mkdir(fullfile(D,F))
end
for k = 1:52
Z = fullfile(D,F,sprintf('Category%d',k));
if ~exist(Z,'dir')
mkdir(Z)
end
end
