Can you attack a planeswalker with a creature that has menace if only one blocker is available?

Menace is an evasion ability that states a creature with menace can't be blocked except by two or more creatures (CR 702.110b). If the defending player controls only one creature, they cannot legally assign that single creature as a blocker for the menace attacker.

Attacking a planeswalker is perfectly legal — when you declare attackers, you choose each attacking creature's target as either the opponent or a planeswalker that opponent controls (CR 508.1). Planeswalkers are valid attack targets just like players.

The defending player still has the option to block with their one creature if it meets all other blocking requirements — they just can't block the menace creature specifically. If they have no other creatures, the menace attacker will deal its combat damage directly to the planeswalker, removing that many loyalty counters (CR 306.7).

Example: You attack your opponent's Liliana planeswalker with a 3/3 creature with menace. Your opponent controls only one creature. They cannot block your attacker. Your 3/3 deals 3 damage to Liliana, removing 3 loyalty counters.

If the opponent had two or more creatures, they could legally block the menace attacker by assigning at least two of them as blockers, potentially saving the planeswalker (CR 509.1).