First of all you need to define ImageFolder, which you have not done. So assign it to some folder on your drives and that will fix that.
Next you need to give a base file name (a string) to full file, NOT an image array.
Or try this snippet:
% Get the name of the file that the user wants to save.
% Note, if you're saving an image you can use imsave() instead of uiputfile().
startingFolder = userpath % Or "pwd" or wherever you want.
defaultFileName = fullfile(startingFolder, '*.png');
[baseFileName, folder] = uiputfile(defaultFileName, 'Specify a file');
if baseFileName == 0
% User clicked the Cancel button.
return;
end
fullFileName = fullfile(folder, baseFileName)
imwrite(img, fullFileName);
