Screen sharing option missing from Ubuntu 18.04 (vino installed)

This question is old, but here's an answer:

Log out, and then start the login (select your user name) but before you enter your password, click the gear-icon...then select the "Ubuntu with Xorg" and NOT the "Ubuntu" option.

Log in, and go back to Sharing...you should see "Screen Sharing" there now.

Screen sharing requires vino package installation per Ubuntu documentation.