Buy on credit card? Initiate a dispute on the transaction with the credit card company. Reason is “Goods not received”, have Roblox account screenshot showing history of debits/credits, purchase receipt, and email correspondence saved as evidence, attach it if requested. Usually you’ll get your momey back without much hassle.

Buy on debit card? Contact your bank and start a similar process but it will be more difficult. Microsoft gift card? Register a complaint with your consumer financial protection organization, depending on your country.