To work with photos, Lightroom needs to import the original images from your camera roll or attached camera/SD memory.
If you were a paid subscriber, the photos would sync to cloud storage and you could then clear them from the application cache. In that same view you shared, there is a 'Clear Cache' button paid users see that clears cloud-synced images from the LR app cache.
Free users of the app do not sync, and so there is no Clear Cache option as this would break Lightroom's ability to control and edit imported photos.
To recover device storage, free users can delete unwanted photos from Lightroom. I recommend you check that you have backups of all your wanted photos outside of Lightroom before deleting anything.