Hi Daniel,
If the Microsoft/Xbox order shows as completed but the Robux has not appeared, first make sure the Xbox account used for the purchase is linked to the correct Roblox account. Then restart Roblox and sign back in.
If the Robux is still missing, check your Microsoft order history to confirm the payment was completed. If it was, contact Roblox Support with the Microsoft order number and purchase details, as Roblox handles Robux delivery to the account.
Thank you, and feel free to respond back for more assistance.