Thanks for joining us in this thread, Copakewillow.

Let's try using an incognito browser to determine the cause of this issue. I'm here to help get you back in business in no time.

It could be that a webpage is preventing you from editing and adding a text message on the invoice template. For further isolation, follow these steps:

• For Internet Explorer of Firefox, press Ctrl + Shift + P.
• For Chrome, press Ctrl + Shift + N.
• For Safari, hold down Command + Shift + N.

If you're able to add a message, clear the cache of your regular browser. This allows QuickBooks Online (QBO) to function more efficiently. 

Here's the link for your reference: Clear my browser cache and temporary Internet files.

That should do it. Please let me know how it goes. I'll be here to keep helping. Take care.