Quick answer
Не волну́йтесь.
ne volnúytes — imperfective, because negative commands almost always are
The options
The negative imperative
A negative command uses the imperfective: не волну́йтесь, не спеши́те, не забыва́йте, не говори́те. A negative *perfective* imperative exists but means a warning — не упади́! means “mind you don’t fall!”, not “don’t fall habitually”.
Which reassurance when
Не волну́йтесь is for nerves and anxiety. Не пережива́й is warmer and more personal. Ничего́ стра́шного answers a mistake or an apology. Всё бу́дет хорошо́ is the big one — the sentence on half the fridge magnets in Russia.
What you hear back
Typical mistakes
Не поволну́йтесь.
Не волну́йтесь.
Negative commands take the imperfective.
Не волну́йся — to a stranger
Не волну́йтесь
The polite form takes -тесь.
Check yourself
Which aspect does a negative command use?