For anyone that has not received e-certificate yet, these are the steps I followed (after googling and looking through multiple forums). FYI I had my oath on Nov 14, 2023 and sent the signed oath form back the same day. However, I did not get the certificate till today Nov 29, 2023. I believe the officials don't receive the initial signed email that you send for some unknown reasons. Nevertheless, here are the steps I followed. Not sure if it would help your case but it definitely did mine.
1. Go to this link
https://secure.cic.gc.ca/ClientContact/en/Application
2. Select 'Add a document to your application' and continue to the form
3. Fill in the details and under what did you apply for select - citizenship and under what application did you submit - grant
4. Under tell us about your request - yes, you asked me to add the document(s)
5. Under the please explain box let them know that it has been more than 5 business days since you submitted your oath and you haven't received your e-certificate and that you have attached a copy of signed oath document (see step 6 below)
6. *Important* re-attach your original submitted signed oath document
7. Review and submit your request
In my case, I received the e-certificate the next day. Good luck!