Hey there! Nice to meet you!
Unfortunately, this only means that the Robux gift card may have been temporarily out of stock in your region. You will need to wait for the Rewards team to replenish the codes.
Microsoft Rewards does not disclose when out‑of‑stock gift cards will be restocked, since there is no defined timeframe.
In the meantime, I would recommend saving your rewards points and checking the Rewards Store from time to time.
You can also provide your feedback regarding this on https://rewards.bing.com/
Click "Provide Feedback" at the bottom left corner of the page > then submit your feedback.
Thank you for your understanding! Please let me know if you need anything else.
Best regards,
Darhyl