Saving Failed

No you’re not missing a plugin. The theme doesn’t need any additional plugins.

I’m almost 100% certain the issue is not related to the Aeonium theme itself.

The first thing to check is to rule out the possibility that it could be a plugin issue.
I would very much recommend that you disable all your currently active plugins, then check to see if the issue is still present.
If there is no saving failed issue, then re-activate your plugins one at a time, checking for the problem each time until the issue re-appears. This will show you which plugin, if any, is responsible.
If the issue is still present with ALL your plugins deactivated, please let me know and we’ll look at next steps.

If I deactivate all plugins, I get a critical error and my website crashes. I went through one by one and deactivated them individually, and still encounter the same error.

Can you try switching to a different block theme such as the default Twenty Twenty-Two theme. Does the issue still exist with other block themes?

The issue does still exist with other themes. Guess it’s not a theme issue after all. Any idea where to go from here?

Thank you for your help. I really appreciate it!!

You could ask in the Fixing WordPress Forum.

Be as specific as you can with details of the error and where you see the issue.
For example is the issue in the site editor when editing templates or global styles, or the post/page editor when saving content.
Also mention that you are using a block (full site editing) theme, as the WordPress editor is quite different to when a classic theme is active.