Yes. As long as you have met the residency obligations and all citizenship requirements, you can apply for citizenship even if your pr card has expired.
It clearly states "You do not need to have a PR card to apply for citizenship. If you have a PR card, but it is expired, you can still apply for citizenship."
I sent my Citizenship application while I was still waiting for my PR card to be renewed. Sent my Citizenship application in Jan 2017. My PR card expired April 2016. As you can see from my times below, it was a very quick process, quicker than my PR card renewal.
I applied June 2015 and my PR card expired soon after (or was it before?? I really can't recall exactly)..... But I took my test,interview and oath,all with an expired PR card.....no problem at all. No RQ,No FP....took 9 months processing.Called IRCC call line to ask and verify,agent told me as long as I don't have plans to leave Canada,it's fine to have an expired PR card during citizenship processing. IRCC officers didn't even bat an eye when I told them my PR card is expired.