Hijacking top comment for an alternate method if, like me, you want to avoid getting your oily fingerprints on your nice new OLED screen whenever possible:
In these cases where the Deck is telling you to use the touchscreen, you can always hold down either the Steam button or the Quick Access Button (three dots) and drag your thumb over the left or right trackpad to move the mouse cursor. Then press Right Trigger to click mouse.
Hijacking top comment for an alternate method if, like me, you want to avoid getting your oily fingerprints on your nice new OLED screen whenever possible:
In these cases where the Deck is telling you to use the touchscreen, you can always hold down either the Steam button or the Quick Access Button (three dots) and drag your thumb over the left or right trackpad to move the mouse cursor. Then press Right Trigger to click mouse.