If you\’re getting this error after adding Membership Page Listing element, please ensure that the child pages is set up as a category or content/lesson page, and the page you are listing it on is set up as a product page.
Please refer to this link below for tutorial in setting up up Membership Content pages:
https://optimizepress.zendesk.com/hc/en-us/articles/204597368-Create-Your-Membership-Content-Pages-