Good day, thanks for your question. There might be something happening on the backend that is causing this, you can check and try it out again after a couple or few hours, and if it is still the same then the next option is to directly purchase from the website via the links below.
https://www.roblox.com/premium/membership
https://en.help.roblox.com/hc/en-us/articles/36...
If you are still looking into using the Microsoft credits that you have, you can try to reach out to the internal team via the link below if you will still encounter an issue.
- Go to this link https://support.microsoft.com/en-us/home/contac... or or you can also launch the Get help app on your PC.
- Type in talk to support.
- Click on the blue button on the below part.
- Sign in/Log in when prompted and try to connect to an agent/support from there and please take note that it might take several attempts before you can get a hold of someone.
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Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.
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Regards,
Paul R.