[Theme; Suffusion] Why is not "Edit CSS" option available in "Appearance" menu?

Try asking on the theme’s dedicated forum: https://www.aquoid.com/forum
You may get a faster & more informed response there.

It shows “You son’t have enough permission”. why?
Anyway thanks.

Suffusion does not add an option to edit its CSS directly, because any update to the theme would potentially clear your custom CSS and JS.
BUT there are a couple safe ways to edit Suffusion’s CSS.

1. Suffusion Custom Styles

Suffusion Options > Back-end > Custom Includes > Custom Styles
You can add your own custom CSS here, or even provide a URL for an external custom stylesheet.

2. Suffusion Child Theme
It’s dead simple to set up, and if you’re doing a lot of customizations, it’s the best way to go.

WordPress Documentation:
https://codex.www.ads-software.com/Child_Themes#How_to_Create_a_Child_Theme
Suffu-Scion Setup Tutorial:
https://aquoid.com/forum/viewtopic.php?f=8&t=6464