Go to the Local Search Rank Tracker page in your account - here's the link https://tools.brightlocal.com/seo-tools/admin/lsrc.
Click on the cog icon as marked in the screenshot below.
Select which search engines you'd like to activate SERP screenshots for. Then hit save.
Congrats, SERP screenshots are now activated with your account! You'll, now just need to rerun your reports in order to see the screenshots.