User advertising is, except for a couple very narrow exceptions, not allowed on wordpress.COM. For the exceptions, see the last sentence under “Affiliate marketing blogs” in the bottom set of bullet points in the “types of blogs” link below.
In short, advertising can get your site suspended or deleted.
If you need to advertise, you will need to self-host a wordpress site on a third-party hosting service such as godaddy, bluehost, etc., using the software from wordpress.ORG. Figure about $10 to $25 per month all things considered and depending on bandwidth and storage requirements.