'01.txt' and '1.txt' are two different filename. You ask to load the latter when it's the former that exists.
file = sprintf('%02d.txt', k);
would fix that issue.
However, I am not aware of an importdata that works on a datastore. So your code is going to fail on the next line. You seem to be mixing two completely different things, datastores and importdata. You need to use one or the other.
Finally, rather than hardcoding the count of files and the filenames, which innevitably you'll get wrong as you've just done, why not simply ask matlab what they are?
folder = 'C:/Users/..../...Recognition2'; %can't copy paste from a screenshot, you'll have to type the correct path
files = dir(fullfile(folder, '*.txt')); %get list of text files in the folder. Guaranteed to be correct with no typo!
mydata = cell(1, numel(files));
for fileidx = 1 : numel(files) %number of files guaranteed to be also correct.
mydata{fileidx} = importdata(fullfile(folder, files(fileidx).name));
%...
%Of course, doing mydata{fileidx} = y; after the previous line completely overwrite what has just been loaded
end
