If a user has Signed up for an Onsite course or Webinar and they cancel their Sign up for some reason, they are not able to sign up for the course again.

They have to reach out to their platform Admin, who can then navigate to admin > learning > participants list >> find the specific schedule and click on the Hat to Manage Participants List. Here they will find a tab for Cancelled and can simply select and delete the user. Once this is done the user will be able to Sign up for the course again.