A program that grants points for using Bing search, which users can redeem for gift cards, sweepstakes entries, or charitable donations.
Hello Md Tahir Hossain Nihal, welcome to Microsoft Q&A forum.
Unfortunately, League of Legends gift cards on Microsoft Rewards go out of stock pretty regularly due to high demand, and Microsoft doesn't announce when they'll restock—could be days, weeks, or even longer. Your points are safe while you wait though.
If you don't want to wait, you could redeem for an Amazon gift card instead and use it to buy Riot Points on Amazon. Amazon cards are always available on Rewards and you actually get slightly better value that way. Once you get the Amazon card, just grab a League of Legends gift card from Amazon and redeem it normally.
Otherwise, you can keep checking the Rewards catalog periodically to see when it comes back in stock.