If the checkout box / payment form for Atom doesn't load in your browser it likely has to do with the software version you have installed.
If issues still persist please try loading the Atom checkout page in either:
A different browser entirely (for example, try Google Chrome instead of Firefox)
Updating your browser to the latest / most recent version.
Links to update the common browsers can be found below:
https://support.mozilla.org/en-US/kb/update-firefox-latest-release
https://support.apple.com/en-us/HT204416
https://support.google.com/chrome/answer/95414?co=GENIE.Platform%3DDesktop&hl=en
If the payment method you're using to purchase Atom is being declined the issue could be that you are trying to pay with a prepaid card and trying to purchase through the payment plan.
Atom's payment plan checkout method is not purchasable with prepaid cards, and thus you'll need to use a different payment method to purchase an Atom license through the payment plan.
Prepaid cards can be used however to purchase an Atom license for full price.