Hi,
Thank you for writing to Microsoft Community Forums.
I understand that you are facing issues while purchasing Robux through Microsoft Store on your PC.
I would suggest you to refer the article Troubleshoot problems when paying for something from Microsoft Store and see if that helps.
However, we do have a dedicated team who can assist you with the issue. Let me help you in pointing towards the right direction, I would suggest you to get in touch with the Accounts and Billing team for further assistance with the issue.
Regards,
Nikhar Khare
Microsoft Community - Moderator