unable to load photo

There are a few things you can try to fix the error "An error occurred while loading a higher-quality version of this photo" on your iPhone:

1. Restart your iPhone. This can sometimes fix minor software glitches that can cause problems with loading photos.
2. Check your internet connection. Make sure you're connected to a Wi-Fi network or have cellular data enabled. If you're not connected to the internet, you won't be able to load high-resolution photos.
3. Update your iPhone's software. Apple often releases software updates that fix bugs and improve performance. If you're not up to date, you may be experiencing the error because of a bug in your software.
4. Force quit and relaunch the Photos app. This can sometimes clear up temporary glitches that can cause problems with loading photos.
5. Reset your network settings. This will erase all of your saved Wi-Fi networks and passwords, but it can sometimes fix problems with your internet connection.

If you've tried all of these steps and you're still getting the error, you may need to contact Apple support for help.

Here are some additional tips for troubleshooting problems with loading photos on your iPhone:

• Make sure that you're not in low power mode. Low power mode can prevent your iPhone from loading high-resolution photos.
• Make sure that you have enough storage space on your iPhone. If your iPhone is running out of storage space, it may not be able to load high-resolution photos.
• Try loading the photo in a different app, such as the Photos app on a Mac or PC. If you can load the photo in another app, then the problem is likely with your iPhone.

Where are you loading it?

Your iPhone maynot have enough space to store all high-resolution photos and videos from iCloud.

Please compare 

Settings>your name>iCloud>Manage Storage

And

Settings>General>iPhone Storage

This will give you a fair idea of how to manage your device storage.

Some of the links below may help

1. Manage iCloud Photos iCloud Photo Library Help - Apple Support 
2. Manage your iPhone Storage --> Manage your photo and video storage - Apple Support
3. Manage your iCloud Storage → Manage your iCloud storage - Apple Support