Under the current rules you cannot apply for an extension. If you look at the policy announcement, you'll see that you need to meet specific criteria in order to qualify for the extension. Unfortunately your situation does not meet this criteria because your PGWP is still valid into 2022.
It's possible IRCC may extend the dates on the policy in a way that allows you to apply. But of course far from guaranteed and none of us here can say if that will happen.