You cannot directly send Robux to another user
Buy a Roblox Gift Card and give them the code
Use the Roblox Group payout system if you own a group and have enough funds
Buy an item from their game or catalog listing if they created one
Purchase a game pass or developer product from their game if they set one up
Use a private sale of an item you own through the Marketplace, if applicable
Make sure the recipient has a valid Roblox account and can redeem the funds or item properly