You don’t really need Jetpack to edit custom CSS on your site. WordPress supports this out of the box
You can head to Appearance -> Customize where’ll you see a section to add Additional CSS:
You need at least WordPress 4.7 and the edit_css capability to see this section. By default this capability maps to unfiltered_html, which only admins (or super admins on multisite) have.
The Custom CSS module in Jetpack improves that feature a little bit and changes that capability mapping to edit_theme_options (still something only admins have by default). Of course you need to activate it first.
When you don’t see an “Additional CSS” section, make sure you’re an admin and try disabling all plugins and switching to a default theme like Twenty Seventeen.