This can happen if the Review section is a Custom Questionnaire, which is linked to an Onboarding Wizard Step that has been removed from the Onboarding Wizard.

If this has happened, the link between the Review section and Onboarding Wizard Step will need to be re-applied. Check that the Onboarding Wizard Step is enabled/recreated.

Then navigate to the Review Section configuration and select the correct Onboarding Wizard step in the dropdown menu.