Since Linux comes in various distributions, there are several ways to take a screenshot, some of which may not work (in this case, try another one).
On Ubuntu and Linux Mint, you can use the GNOME desktop environment where the Print Screen key works.
To take a screenshot of the entire screen, click the button PrtScr. After creating a snapshot, you will be prompted to select a directory in which it will be saved.
To take a screenshot of the current window, use the keyboard shortcut Alt + PrtScr.
To take a screenshot of the selected area of the screen, simultaneously press the keys ⇧ Shift + PrtScr.
You can make a delay when generating a screenshot, as well as display a cursor or a frame on it, this all changes in the settings of the Gnome-Screenshot utility.
Several Linux distributions support the GIMP graphical editor. Try to install it, you can find it in the application center under the name "GIMP Image Editor".
After installation, launch it. Click "File" - "Create" - "Screenshot". Choose the appropriate settings (for example, "Delay" - the time delay from the moment of launch to the screenshot). Click on "Snap" to take a screenshot, and it will open in a new GIMP window for editing and saving.