You CAN use ( احاطه ) if you want to say what you mean, and Persians will understand what you are saying but when we want to talk about fluency in a language we use the word "تسلط" .
Then your sentence will be;
من هنوز به زبان فارسی تسلط کافی ندارم.
(Or ; من هنوز به زبان فارسی مسلط نیستم)
"تسلط" is an Arabic word but we use it too, that's why the second sentence I wrote is a little bit different. Here, «مسلط» is called " اسم مفعول" in Arabic. the meaning is somehow as same as «مفعول» in Persian.
So you can use these 2 sentences.
Hope this helps.