This happens when there is an existing redirect to the current page. You will need to remove any redirects before moving the page to a landing page, using the following steps:
  1. Navigate to onMessage > Settings > Website Redirects.
  2. Search for the page.
  3. Click the red X next to any redirects that list the page as a destination.
  4. Click Confirm.

Once you have converted the page to a landing page, you can create new redirects that use the page as a destination.