Hi ict18,
I am in a similar situation and in my case I had to apply for BOWP soon as well. Hence, I had called the IRCC helpline and was told by an agent that I do not have to wait till the last day of ITA expiration. Like others have mentioned here on this thread, submission of proof with letter of explanation mentioning that you have made your best efforts to obtain one and would provide the same as soon as you receive it.
In my case, I had submitted multiple copies of finger prints and in a second mailer to FBI, I sent digitally scanned fingerprints printed on a card as well. I have provided proof of all of these with an LOE. The background check process will not be started till they receive the PCC. But you should be OK with submitting the application with LOE and proof (at least that's what I was told by IRCC).
Hope this helps.