Can a player attack a planeswalker that entered the battlefield this turn?

A player may declare any planeswalker controlled by an opponent as an attack target during the declare attackers step, with no restriction based on how recently that planeswalker entered the battlefield (CR 508.1). The rule that prevents a permanent from attacking or using tap-symbol abilities — commonly called 'summoning sickness' — applies only to creatures, not to planeswalkers (CR 302.6).

Planeswalkers are not creatures (unless a card effect makes them so), so CR 302.6 simply does not apply to them. A planeswalker that just entered the battlefield this turn is a perfectly legal attack target the moment it is on the battlefield, even during the same combat phase of the very turn it arrived.

When you declare attackers, you choose each attacking creature and announce whether it is attacking a player or a planeswalker an opponent controls (CR 508.1). As long as the planeswalker is on the battlefield and controlled by an opponent, it is eligible to be attacked.

Concrete example: Your opponent plays Liliana of the Veil on their turn, and you have no instant-speed removal. On your next turn, you can immediately swing your 3/3 creature at Liliana, dealing 3 damage to her and removing 3 loyalty counters — her having just entered the battlefield provides her no protection whatsoever.