air drop missing from finder

Restart both devices and check again.

Make sure both devices have Bluetooth enabled.

Apple’s guidance for AirDrop is here: Use AirDrop on your Mac - Apple Support

What sidebar items are missing?

Have you enabled these items in Finder > Settings… > Sidebar?

If the missing items are folders, have you tried dragging the original folders into the sidebar to add them again?

Customize the Finder sidebar on Mac - Apple Support

If the issue is just Airdrop then review this discussion. Many (including myself) have reported issues keeping Airdrop in the Finder sidebar.

Cannot add AirDrop to Finder sidebar in S… - Apple Community