There could be a couple different reasons.
Firstly, it depends on your location. Retale is only available to U.S. users so your account must be registered in the U.S. If you are accessing Retale from a foreign account or if you have a foreign credit card listed on your account it could prevent you from downloading the app.
Make sure that you have enough storage on your phone and that you have a good internet connection before trying to download.
If you still are having trouble, check with your carrier for other solutions or contact our support team at firstname.lastname@example.org