Hello Stephen435.
Thank you for using Apple Support Communities and for sharing these details with us. It sounds like you're getting prompted "photos and more will be shown once Messages finishes indexing". However, it never completes. We can understand how this would be an issue. Allow us to investigate this issue and assist in any way we can.
First, we'd like to recommend some basic steps you can try. Force close the Messages app and then restart your iPhone:
Close an app on your iPhone or iPod touch - Apple Support
Restart your iPhone - Apple Support
If that doesn't help, you'll want to make sure your device is fully updated on its iOS version. If you do have pending updates, the following resources can guide you through the update process:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
Also, if you navigate to Settings > [your name] > iCloud > Messages, is this feature enabled?
Feel free to keep us posted on your results. Cheers.