Please try the following:
1) Delete the app from your phone: long press the ProCamera icon on the home screen, tap Remove App. (Before deleting the app, please make sure that no important captures are saved in the Lightbox and Private Lightbox folder since files in these folders will be deleted as well).
2) Go to the App Store app > tap the user avatar in the top right corner > scroll all the way down and sign out.
3) Restart your iPhone (https://support.apple.com/en-us/HT201559).
4) Try downloading the app once more. You will need to enter your Apple-ID.
We have heard of sporadic cases where downloading failed at first, but after a while and various attempts it would always work in the end. Please make sure you are connected to the internet.