configuration wizard could not be started

I have deactivated off all of my plugins except Health Check and Yoast SEO; and reverted to the 2017 theme but still cannot run the wizard.

According to the Chrome console, there are two HTTP 500 errors.
1) cannot load https://my-domain.com/wp-json/yoast/v1/configurator
2) cannot load https://my-domain.com/favicon.ico

I do not have a favicon.

why is your plugin looking in a wp-json directory which does not exist.