Microsoft Rewards redemptions and purchases made with them are generally final and non-refundable.
All Microsoft Rewards orders are not eligible for return or exchange once redeemed, except in cases of defective products or where required by law. After redeeming Rewards points for a Microsoft gift card and then using that balance to buy “400 Robux for Xbox,” the transaction is treated as completed.
If the Robux were tied to an Xbox/Roblox account that is not accessible or was never properly created, standard policy does not guarantee a refund or re-issue of the Robux. The only remaining option is to contact Xbox/Microsoft Support directly and ask if an exception can be made, but this would be outside the normal Rewards refund policy.
References: