How to enable the sidebar in finder?

Solution 1:

Carefully move the pointer to the left side of the finder window border till the cursor changes to arrow pointing to the right.

↦

Hold and Drag to the right.

If that did not work you need to restore the finders preferences.

In terminal run

rm ~/Library/Preferences/com.apple.sidebarlists.plist

and

rm ~/Library/Preferences/com.apple.finder.plist

Restart.

For me, the solution is that I need first (by Apple mis-logic) to hit:

"View / Show Toolbar" (⌥⌘T);

…then "View / Show Sidebar" (⌥⌘S) becomes ungreyed.