Hey there! Nice to meet you!
Based on your description, kindly double-check the purchase status on your account on account.microsoft.com > Order History > Select the date range when you purchased Robux.
If your transactions status show as unsuccessful and if you have been charged for it, the payment should be refunded to you within 24 hours.
Important: If you're playing Roblox on a Windows PC and purchased Robux through the Microsoft Store, please note that this will not work. Robux bought from the Microsoft Store is only valid for the Roblox Xbox console version. To use Robux on PC, you'll need to purchase it directly from the Roblox website: https://www.roblox.com/upgrades/robux
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.
If you haven't been refunded from your purchase, please reach out to Microsoft Support for further assistance, as they have the tools and resources to properly investigate this case. You may contact them through this link:
Type "Billing Issue" > Contact Support > Microsoft 365 and Office > Billing and Payment Updates > Confirm > Chat with a support agent in your web browser
Phone Support: https://support.microsoft.com/topic/customer-service-phone-numbers-c0389ade-5640-e588-8b0e-28de8afeb3f2
Thank you for your kindness and understanding! Please feel free to let me know if you need anything else.
Best regards,
Darhyl