Hello @koraykaan,
One thing to keep in mind, please do not edit the page outside of Cornerstone. It might corrupt the cornerstone data and will no longer allow you to edit it back in Cornerstone editor.
DO NOT make any changes when you are in Post/Page WP Editor. Go to X/Pro > Content and find the post or page you want to edit.
NOTE: I can confirm that the missing “Edit in Cornerstone” (or “Edit in Pro Editor” for Pro theme) is a conflict with the latest version of WordPress 5.5 and the latest release of the theme. Our developers are already aware of this and will be releasing a patch update anytime soon.
Hope this helps.