I would say: すみません。私はまだ日本語が上手ではありません[わたしはまだにほんごがじょうずではありません] / 私はまだ日本語が上手に話せません[わたしはまだにほんごがじょうずにはなせません]。 (I'm sorry, my Japanese isn't good yet / I can't speak Japanese well yet.) Sometimes changing the words when we translate makes the sentence sounds a bit more natural in Japanese. I hope this was helpful.

This isn't a direct translation of what you have, but it gets the same idea across. Usually when you express something you are not good at, you would use the word 苦手（ながて）, which means "not good at" in the context of a skill. So here is how I would say what you want to express. すみません、私の日本語が苦手ですが。 You could also use the word 下手（へた） instead, but I think it sounds a bit stronger.