A lot of people start in sales. The work is hard compared to client service or campaign management, so the job is easier to get. Pay might be low if you have a hard time closing, but you’ll learn the language and interact with clients and campaign managers, which will make it easier to transition into a campaign management role in a couple years if that’s still what you want then.
People who can sell new clients will always make more money and have more job security in an ad agency than people who can’t.