Errors while trying to buy Minecraft

Even if this question has an accepted answer, let me explain what those error messages mean.

First, I'm not that good on Javascript (the programming language that the page uses) and React (the framework being used).

Uncaught Invariant Violation: Minified React error

From StackOverflow, this indicates some formatting problem on the page source code. Nothing that you can do from your side.

Try to clear your browser cache.

Failed to load resource: the server responded with a status of 401 (Unauthorized)

The 401 HTTP code indicates that you are not authorized to see a specific page. In this case, accessing the Mojang API (https://api.mojang.com/user). If you can log in, start with that.

If you keep getting this error, clear your site cookies.

Cross-Origin Read Blocking (CORB) blocked cross-origin response with MIME type application/json. See https://www.chromestatus.com/feature/5629709824032768

SecurityError: Permission denied to get property "href" on cross-origin object

The page tried to load a JSON file from another website (Microsoft.com) and Chrome/Firefox didn't allowed it for safety reasons.

Failed to load https://api.mojang.com/user/profile/agent/minecraft/name/Kerndog73 resource: the server responded with a status of 404 (Not Found)

The page tried to check if your username is available, and it is. This should not affect the page.

Failed to load https://api.mojang.com/user/profile/agent/minecraft/name/Kerndog73 resource: the server responded with a status of 400 (Bad Request)

The page tried to check your username again but did the request incorrectly and caused the "You need to enter a profile name" message.

XML Parsing Error: prefix not bound to a namespace

The SVG image is not valid. This does not affects the page other than cosmetically.

This often happens when your payment method is not available.
You could try to add your CC to Paypal and try paying using Paypal.