Just tried it again and go the error.
1. Clicked on Edit of the QBO_Invoice(2) screen.
2. Clicked on Edit of the InoviceLineItem table
3. Clicked Next to get to Step 2
4. In the bottom-half section under "Edit an Existing Colum". Under caption I delete "Rate" for field name Rate and change to "Amount". Click return and get the error message. (I get the error message not matter what caption I type in.
I've already changed the caption for Amount and it worked fine. I've also done this very procedure in a separate account and it worked fine. Not sure why I'm getting the error message.