To resolve this, perform the following:
1. Use a Standard Event Form. If the ability to edit is present on a standard form, check the Custom Event Form's settings.
- Navigate to Customization > Forms > Entry Forms
- Click Edit on the Custom Event Form in question
- Click the Fields tab > Message subtab
- Ensure that the Display Type for Message is not set as Disabled
2. If still grayed out on a Standard Event Form, try a different browser.
3. Clear browser cache if able to edit the Message tab when on a different browser.
Note: Steps above can be performed when the issue happens on an existing event. However, Permission and Event restrictions (e.g. Calendar preferences) have to be considered as well.