May I ask you to use the "code" function of this website to add your code instead of a picture. It makes it easier for us to copy your code if necessary.
In your code, I see that the filename of your output file is t/a.txt. Then, except if you have a folder named t, it would not work. So my first suggestion would be to get the real path:
writetable(T, fullfile(t, 'a.txt'), 'Delimiter', ' ', 'writevariablenames', false);
Second, the line with fullfile in your code is useless because fullfile is supposed to be used to merge a full filename path. So you can skip it.