LockDown Browser does work with Chromebooks, but a little different than a traditional Windows or Mac system.  Here are some quick instructions that might help for the future:

Installing the Chromebook Extension:
To install the new extension to a student-owned Chromebook, students will need to use the regular LockDown Browser installation link for your school (they will NOT be able to search for the app directly from the Chrome web store.)  The LockDown Browser install is located at the bottom of the Quiz. The LockDown Browser installation link will direct students to the Chrome web store, at which point they should select “Add to Chrome” to install the LockDown Browser Extension.

Also to note that you may received a pop-up notification in the address bar of the browser when you attempt to launch an exam.  When you click the notification, it will ask if you want to enable pop-ups for mccs.brightspace.com.  Allow pop-ups and relaunch the exam.  It should then load right up. 

If they aren’t receiving the pop-up notification have them open the Chrome browser (web browser), and go to "Settings" >  "Privacy and Security" > "Site Settings" > "Pop ups and redirects" and move the “Allowed” toggle to blue: this will allow popups