How do I remove the ad of the jotform under the page?

Only paid subscribers have an option to remove the JotForm Branding. To upgrade your account, please follow this guide: https://www.jotform.com/help/331-How-to-Upgrade-My-Account

After upgrading your account, go to this link: https://www.jotform.com/myaccount/settings and disable the JotForm Branding.

I hope that helps. Thank you.