Problem: In ChromeTools, thumbnails of student screens never display, but all of the teacher tools seem to function. Device view will properly show the list of open tabs and allow them to be closed remotely, but no screenshot appears here either.
Possible Solution #1: Screen viewing permission may be turned off in Control Tower for the role that's attempting to use the feature. To resolve, go to Settings > Permissions and check the "View Screens" permission setting for each role.
Possible Solution #2: Screenshot permission may be disabled for Chromebooks in Google Admin Console. In Google Admin, go to Device Management > Chrome > User Settings. Find and select the OU that contains the student devices that exhibit the issue. In the "search settings" box at the top type "screenshot" to find the setting pictured below, or scroll down about 2/3rd of the way to the Content section and look for the Screenshot setting. Set this to "Enable screenshot" and save your settings. If the setting is inherited, you may wish to navigate to a higher OU where the setting is inherited from and change the setting there instead.