Just as the title says..and you shouod really know that anyways, since it was one of the things you agreed to not do when you signed up..
But..you can advertise if you buy the advertising upgrade (or something like that), and you would be able to serve ads via WordAds. I think you can be able to put up almost anything (like ads) if you upgrade to business plan or something.. I’m not really familar with it..
I’ll post a few links up later that you should read and understand if you are a WordPress.com user. WordPress.org blogs have greater freedom, but it comes with bigger responsibilities. If you sant to do more, you can also make a blog at Weebly, Wix, Blogspot, or other platforms.