Board Game Arena

I am in a game where I got an aggression against me but it seems I cannot play defense cards. What is wrong here?

It says I can play or discard cards but when I press on my defense card it still only says discard and not play.

The button is misleading and should said "Discard or Play", not "Discard". You're suppose to select all the cards you want to discard or play, and click on "Discard".