str2num won't work with a word ("sub"). It's also unnecessary if you are using sprintf.
One other correction I would make is in how you preallocate your array. The empty brackets initializes your variable Sub as a double, so all strings will appear as NaN. You need to initiallize as an empty string.
Try something like this.
Sub = strings;
for i=1:50
Sub(i,1) = sprintf("sub%02d",i);
end
% view result
Sub
