Can a planeswalker be attacked the turn it enters the battlefield?

Summoning sickness (CR 302.6) only restricts creatures from attacking or using tap/untap activated abilities the turn they enter the battlefield under your control. Planeswalkers are not creatures (unless a card says otherwise), so that restriction does not apply to them at all.

Instead, the relevant rule is CR 508.1, which defines legal attack targets. An attacking player may declare attackers against any player or any planeswalker an opponent controls (CR 508.1c). There is no clause requiring the planeswalker to have been on the battlefield for a full turn cycle.

The defending player does not choose whether to 'block' the planeswalker — the attacking player chooses to direct attackers at a planeswalker. Blockers then block those attackers normally, and any combat damage that gets through is dealt to the planeswalker, reducing its loyalty counters (CR 510.2).

Concrete example: Your opponent plays Liliana of the Veil on their turn, activating her +1 ability. On your very next turn you can immediately declare one or more of your creatures as attacking Liliana — she has no protection from being attacked just because she entered this turn.

Note: if a planeswalker also happens to be a creature (e.g., via a type-granting effect), summoning sickness would prevent it from attacking, but opponents can still attack it as either a creature or a planeswalker regardless.