Yes, of course you can.
But there are some "rules" to follow when monetizing your website on WordPress.com.
It depends on the way you want to monetize your website.
Only the Premium ($8/m) and Business ($25) plans allow using ads networks as follow:
As any explicit rule of any kind, trying to get around it will result of warnings or ultimately banning you as they see fit.
Make sure you check their monetization rules’ page.