I'm playing the Lord of the Rings Saga: Black Riders box. The Flight to the Ford campaign card of the third scenario has a response that says:

" Response: After an enemy is declared as an attacker, shuffle the top card of the burden deck into the encounter deck to cancel that attack. (Any player may trigger this ability.) "

The Witch-King's enemy is " Immune to player card effects. "

Can I use the campaign card's response against The Witch-King?

Thanks.

The campaign card is not a player card. I'm not sure what its official designation is, but it isn't a player card. "Player card effects" refers to effects from player cards, rather than card effects activated by a player.