Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Sorry to hear you're having trouble.
First, please double check https://account.microsoft.com/billing/orders to see if the payment did actually go through. If it did not then you can double check the payment information at the same site. Also, if you aren't already doing so please try using the Edge browser if you haven't done so already. You can also try using an Incognito Window within the browser in case it's an add on that's causing the trouble.
Beyond that, if you still cannot make the purchase please start an online chat with support via https://support.microsoft.com and click on Contact Us.