If you are getting this error on FAQPage markup, it means that you haven’t introduced the Questions and Answers in the FAQ section in Schema plugin post meta.
This triggers an error in Google Rich Results testing tool:
To fix this schema.org FAQPage markup error, you will need to add the Questions and Answers in the Schema post meta fields.
So, try to edit your post or page which has the FAQPage schema enabled, and scroll down to find the Schema post meta fields and your questions and answers like shown in the screenshot below:
It’s not enough to have them presented in the post content since the plugin will not add them automatically. Questions and answers should be added manually in the Schema post meta fields.
After you are done, save your page and test the schema markup again to make sure all is good and the error has been cleared.