I’ve been trying to reinstall some default apps I had previously deleted such as “Watch” and “Files” through the App Store however with all of them I get the message “Unable to install ‘app name’ please try again later”. All other apps download fine, it’s just the default Apple apps that won’t install.
Try restarting your iPhone. Sometimes rebooting it , helps.
I found that resetting all settings in Settings>General>Reset sorted the problem out.