From my understanding, you are trying to edit an end user notification on the defender portal. It is possible to create new and edit Tenant notifications. Here is a link https://learn.microsoft.com/en-us/defender-office-365/attack-simulation-training-end-user-notifications#modify-end-user-notifications
I am unable to replicate your scenerio as I have the ability to edit and create end user notifications from the portal.
- You can use the import email button which activates choose File to select a new file (txt) which can replace the exisiting notifications.
- You can change the content by selecting text and adding any required text on the notification.
- Finanlly toggling to code allows for you to view and edit exisiting code.