Hey there, Dxd8C
If you are having issues purchasing content on the Microsoft store, please post this question on the Microsoft Community here: https://answers.microsoft.com/en-us/windows/forum
Please select the option to ask a question found above the thread listings
The community over there should be able to offer some troubleshooting steps to resolve this problem you are facing.
It may also be worth contacting Roblox support via this link for them to be able to help you. To reach out to their support team, please use the following link: https://en.help.roblox.com/hc/en-us/articles/208448546-Contact-Us
As this question is off topic here, I will update the tags in this question to not monitored.
Hope you can get this problem resolved,
Nathan