Their system upgrade got screwed up and they are still in process of fixing it. All you can do from your side is call them and ask them if you really need to submit any document or not. You can also open a technical issue.
I opened a technical issue a week back and this is what they replied. I opened another technical issue with them today regarding "Additional Document" status. Hopefully they will reply in a week or two.
UN MESSAGE EN FRANÇAIS SUIVRA.
Thank you for contacting Immigration, Refugees and Citizenship Canada (IRCC).
The message that you have indicated in your email is indeed the result of a technical problem that has now been resolved. You should see a new message indicating that, if you need a medical exam, a message with instructions will be sent to you. Please try again.
If you do not see this change, we invite you to fill out the IRCC Web form and include:
· Screen shots of your account including:
o the Welcome Page,
o the sections titled View my submitted applications or profiles and Account messages.
We apologize for any inconvenience this may have caused.
Sincerely,
Immigration, Refugees and Citizenship Canada
www.cic.gc.ca