You cannot perform a Scout action if there are no cards left in the deck.

Explanation

• The Scout action requires you to "reveal the top card of the deck and place (not deploy) it on any location" (page 3).
• In the "Edge Cases" section, it states:
  "Deck runs out: If there are no cards in the deck, the deck simply ceases to be an option for players to draw from or interact with." (page 3)

Since the Scout action specifically requires a card from the deck, and the rulebook clarifies that the deck is not an option when empty, you cannot Scout if the deck is depleted.

Source: page 3

Source: Core Rulebook (pp. 1, 3, 5, 6)