The screenshot creation scheme in Chromebooks (and every device running ChromeOS) differs from the usual schemes in Android and iPad due to the presence of special unique keys.
To take a screenshot of the entire screen, simultaneously hold down the keys Ctrl + .
To take a snapshot of the selected portion of the screen, press ⇧ Shift + Ctrl + . After that, the cursor will change to a cross, which means you need to select the area of the screen that will be included in the snapshot.