Hello robloxplayer,
Thanks for reaching out. It sounds like the Robux purchase went through on Microsoft/Xbox's side but didn't credit to your Roblox account
First, you should check the order status. Go to https://account.microsoft.com/billing/orders and confirm the order status shows Completed. If it's still pending, give it some time.
If it shows Completed, the charge went through on Microsoft's side, which means the issue is on Roblox's end not receiving the credit. In that case, try fully closing and reopening the Roblox app on Xbox (don't log out) and check your balance again. Sometimes there's just a sync delay between Xbox and Roblox's servers.
If it's still missing after that, you should reach out to Roblox Support directly, so they can look into the account credit on their end. You can also request a refund through the Xbox refund form if you'd prefer to go that route.