That usually happens for one of these reasons:
-You're downloading through the Itch app but your device would work better using the direct site. Or you're using the site and should try using the app
-There's not enough space on the device or the device doesn't allow apps not from the app store to be added or your virus scanner thinks it's not common enough to be safe or something
-The internet connection isn't strong enough or is having other issues
-Itch.io the website is having hosting issues
Unfortunately, we can't influence how Itch/the internet connection/the browser/the device handles things, so there's not really anything we can do for you. But I hope you're able to get it to work!