Make sure you submit your application within 60 days of receiving your ITA even if you do not have all your required documents. In this case, you'll have to write a LOE and upload them in the placeholders for each of the missing documents. If you do this, your application would not be canceled for incompleteness.
You will have 90 days more to submit the missing documents. If after the 90 days, you still don't have the documents you'll be given more days - always inform IRCC though through web form that you're unable to get the documents before the 90 days are over.
There is a recent case in this forum of an applicant who thought he had 90 days to submit his application. After the 60 days passed, his application portal was closed and he was unable to submit. He has since created another EE profile and is waiting for ITA again.
I hope this helps.