Unable to download an app using “double click to download” option on iPhone

So I am trying to download an app and whenever I press “get” it gives me the usual prompt of “double click to download” in which I should be able to double click my power button. This would result in me downloading the app, but whenever I did it this time, it doesn’t download the app and it just says get again. I can’t download any app anymore and I want to know how to fix this.

[Re-Titled by Moderator]