Online booking needs at least the Starter plan. On Free, your public booking page still exists and your link still works, but instead of your availability a visitor sees:
Online booking is paused. Contact [your name] at [your email or phone].
That contact line only appears if you’ve added a notification email or phone number in Settings. With neither set, the page just says booking is paused, with no way for a visitor to reach you from that page.
Everything else about your account keeps working on Free; this is specifically about the public booking page. See Manage your subscription for how to upgrade.