Here’s why I can’t toggle “on.” All of the pages that have this problem are Elementor pages, and AMP doesn’t play nicely with Elementor.
When the AMP box is toggled “on,” the Search Console inspection tool gives me an error message which reads “AMP Page is Invalid — It won’t appear in Google search results.”
When the AMP box is toggled “off,” that eliminates the Elementor incompatibility, and the Search Console inspection tool returns a message which reads: “URL is on Google, it can appear in Google search results with all relevant enhancements.”