Do not try to enter the code in the "Coupon" section of the mobile app itself (that section is for live event codes). You must use the official Netmarble Seven Deadly Sins: Grand Cross coupon website.
For the most current April 2026 rewards, you can also try redeeming general active codes like 777OURSPRING (for a Hellbrim selection jewel) or 7DS77THANNIV for other anniversary perks. full list of current April 2026 active coupon codes for general rewards like Diamonds and Draw Tickets? 7ds grand cross 35 anniversary box code work