Why can't I edit the message template subject or body?

Environment:

Message Templates
Oracle B2C Service

There may be other reasons why you cannot edit the message template subject or body, but if you have any custom HTML you have added to your message templates this answer may be applicable. 

Resolution:

The message templates in their simplest form are HTML, but do not meet the same requirements you might need for a website. Email HTML is slightly different and as such, when you try to implement HTML elements like you would a website, it can cause issues. If you are having issues with a single message template and not being able to edit the subject or the body while using the design mode of the message template, check your custom HTML you have added as this is likely where the issue resides. You are always welcome to submit a service request with us, but our process will be the same.

Now, if you are having issues not being able to edit all of your message templates, the issue likely resides with the Global Message Template. The process is the same, view the source of the global message template and review your custom HTML. You can try taking one piece out at time and see what fixes the behavior.

*Note: As you are removing HTML, you will need to deploy and save each time. You will also need to log out of the console and back in order for the changes to be realized within your console.

As for what elements to look at, email HTML is very simple so the common elements shouldn't be an issue. We typically see issues when something like the following is attempted.

For more information on Message Templates, refer to the following resources:
Customizing notifications and email messages sent by Oracle B2C Service
Editing the Global Template