二通り意味がとれると思います。一つは「頑張るな」。
もう一つは例えば"I will try".と言ったら、政治家の「善処します」みたいに、やる気はないのにその場しのぎで言っちゃうみたいな意味になります。その場合は「やろうとするんじゃなくて、やれ!」の意味になるかと思います。言葉を使った状況かと思いますが、彼の人生を考えると、「頑張るな」だと思いますね。
「頑張るな」説もあるにはあったのですが、元々は"My theory of life [is] 'Don't try. Just type'." (俺の人生のモットーは「四の五の言ってないで、とにかく書け」ということだ)という彼の言葉からの引用だと思われるため「頑張るな」は誤訳だというのが通説みたいです。
His gravestone reads: "Don't Try", a phrase which Bukowski uses in one of his poems, advising aspiring writers and poets about inspiration and creativity. Bukowski explains the phrase in a 1963 letter to John William Corrington as follows:
Somebody at one of these places [...] asked me: "What do you do? How do you write, create?" You don't, I told them. You don't try. That's very important: not to try, either for Cadillacs, creation or immortality. You wait, and if nothing happens, you wait some more. It's like a bug high on the wall. You wait for it to come to you. When it gets close enough you reach out, slap out and kill it. Or if you like its looks you make a pet out of it.