答案是：
✘ It's so relax.
✓ It's so relaxing.

放鬆我們很常用 relax，但 relax 是一個動詞，如果我們要說一件事很使人放鬆，我們應該用形容詞 relaxing，例如：I find reading so relaxing.
It's so relaxing to set here.
It's so relaxing to talk with you.

如果我們要說我們感到放鬆，那就要用relaxed. 例如：
I feel so relaxed.
He seems relaxed after knowing the result.