Edit CSS panel doesn’t open

Good day! When I access Appearance > Edit CSS, I get directed to a blank screen at https://www.zacheverson.com/wp-admin/themes.php?page=editcss-customizer-redirect.

Likewise if I try to access the feature via Appearance > Themes > Edit CSS.

My theme is Twenty Twenty-One and I’ve deactivated all plugins except Jetpack (And obviously the Enhance CSS customization panel slider in Jetpack > Writing > Settings is on). Any suggestions how I can access Edit CSS? Thanks

The page I need help with: [log in to see the link]