i can not find the hidden file under utilities in my iPad air

For the hidden folder/album to be visible, you must first enable the hidden album in iPad settings:

Settings > Photos > Show Hidden Album - set to ON

Having enabled the album, for the album itself to be accessible, you must now assign at least one photo to the hidden album… 

From your Photo Library, identify a photo that you intend to hide. Tap the Selectbutton at top-right of the Photo App window - and individually select the photo(s) that you wish to add to the Hidden album. When you’ve selected the images that you wish to hide, tap the ellipsis button (…) at bottom right of the screen. A menu will appear; tap Hide.

Having initially added photos to your Hidden photo album, this album can be directly accessed from the Photos sidebar.

Additional description of using this feature can be found here:

Hide photos on your iPhone, iPad, or Mac - Apple Support

If you turned on the Show "Hidden Album" switch in Settings ➜ Photos, then an Album called "Hidden" should appear in the Photos App, side panel, in the Utilities section if and only if you have hidden at least one photo.

If will not appear if you have no hidden photos.

click here ➜ Hide photos on your iPhone, iPad, or Mac - Apple Support

To hide a photo, open the photo, tap on the (...) icon at the top right corner, tap on hide, and then tap on Hide Photo to confirm.