1) If the share screen window next to cancel is grey, click on the image under "Share your Entire Screen", the share button will turn blue and you can click share.
2) If Step #1 didn't work, refresh the page.
3) If Step #2 didn't work, force quit Chrome and relaunch the exam again.
4) If Step #3 didn't work, complete the steps to create a New Chrome Profile.
Helpful suggested articles for review:
Honorlock Test Taker Toolbar Overview
Honorlock Test Taker Best Practices