This has not been an easy experience, @curra. I acknowledge your effort in doing the troubleshooting steps to resolve the error so you can modify your invoice today. 

Since the issue persists, I recommend contacting the QuickBooks Online (QBO) Phone support team to further isolate this error based on the screenshot you shared. You will have a one-on-one interaction to determine when the error started and will ask for some relevant information. Once the root cause is determined, they can help and guide you on what to do to fix the error. 

Here's how:

1. In your QBO mobile application, go to the Menu icon.
2. Select the All tab and choose the Help option below.
   
3. Tap Ask QB assistant a question.
   
4. Click on Contact Us below.
   
5. In the What can we help you with? section, pick Ask about something else.
   
6. Type in a short description of the error and Continue.
   
7. Choose either Chat or Callback.

In the meantime, you can log in to your QBO account via a web browser and edit your invoice transaction there so you can finish your task today.

I appreciate your gesture in giving me the chance to assist you today with your issue in editing an invoice using the QBO mobile app. Whenever you're free or have more questions, leave a comment below.