Hey there KristinLynners,
When running into this issue, first check to make sure you have plenty of storage available so your messages can sync properly. To check your used and available iCloud storage, follow these steps:
1. On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud.
The bar graph shows your overall storage usage.
2. Tap Manage Storage for more details.
Below the graph, you see a list of apps and features and how much iCloud storage they use. The apps and features that use the most storage are at the top of the list. You can tap an app or feature for more information.
To check storage from another device: Check your iCloud storage online or on any device - Apple Support
If you need to clear up some storage space in iCloud to help make space for your Messages, use the guidance here: Manage your iCloud storage - Apple Support
If you're still unable to select "Undo Disable & Delete" without an error, Contact Apple Support for next steps.
Take care!