Hello san,
I'm sorry you're going through this — that's really frustrating.
Unfortunately, Robux purchased through the Xbox Store (store.xbox.com) is platform-locked to Xbox consoles only. Even though you bought it via browser, it only works on Xbox — not on PC Roblox or the Microsoft Store version. As for the refund — Microsoft's policy for digital currency like Robux is extremely strict and typically non-refundable. These purchases are considered "delivered" once processed, so the system usually auto-denies refund requests.
You could try re-appealing through Microsoft Support, but similar cases have been denied multiple times due to this policy. I wish there was a better option here.