As per their store website:
"You must provide your invoice when returning merchandise to a store. You may return your merchandise to any Forever 21, XXI Forever, or ForLove21 location within the United States for an exchange or store credit. Refunds for online purchases are only processed when the merchandise is mailed to our Online Returns address. Non-Forever 21 branded merchandise may be returned by mail only.

Gift cards, e-gift certificates, cosmetics, underwear, select lingerie, select swimwear, earrings, body piercings, and online items marked as Sale are all FINAL SALE and may not be returned or exchanged."

So yes, you can return in store.