Thanks for reaching out to the Apple Support Community. I understand you’re having trouble downloading a purchase, and I have some information that may help.
If you are trying to download via your cellular network, there is a 100 Mb download size limit. Have a look at the excerpt from the article below for more information. To continue this download, you can do so from a Wi-Fi network, or via your computer and then sync the content to your iPhone.
Turn on Automatic Downloads for iTunes Store, App Store, and iBooks Store purchases - Apple Support
You can turn Automatic Downloads on or off over your cellular network on an iPhone or iPad Wi-Fi + Cellular. Go to Settings > [your name] > iTunes & App Store and turn Use Cellular Data on or off. Cellular Data downloads are limited to a file size of 100 MB or smaller.
If the behavior persists or you run into any trouble, please reach out again.
Best Regards.