。
She isn't very good at cooking.
Literal
She [topic-は] skillfully cooking [subject-が] cannot-do.
A standard polite-register negation built on できません, the negative potential form of する. じょうずに is the adverbial form of the na-adjective じょうず ('skilled, good at'), and 料理ができる is the potential 'can cook' construction — note that 料理 takes が, not を, in the できる pattern. じょうずに料理ができません is a softer way to say 'she can't cook well' than the blunter 料理が下手だ ('she's bad at cooking'); reframing inability as a question of skill level keeps the tone gentler. The hiragana spelling じょうず (rather than 上手) is a stylistic choice that softens the tone further.