Your K740 keyboard's Scroll Lock is on the PAUSE/BREAK key located in the top row just above the PAGE UP key.

To toggle Scroll Lock off or on, press the FN + PAUSE/BREAK key at the same time. The FN key is beneath the shift key on the right side of the keyboard.

NOTE: If you've installed the Logitech SetPoint mouse and keyboard software, the confirmation message "Scroll Lock Off" or "Scroll Lock On" will be displayed for about two seconds in the lower portion of your monitor each time you toggle the key. If you don't have SetPoint installed, you can download it.