Hey @MOuRos_, thanks for getting through to us, welcome to forums and sorry to hear about the difficulties you've ran into when attempting to purchase the game.
First and foremost, in regards to your 300 Club Units that you've lost - I can confirm that these have now been refunded for you, so you're able to re-purchase the discount code, if you wish.
As far as troubleshooting goes, can I ask if you've possibly tried clearing cache/cookies on your browser or using an alternative one (can also consider trying on Incognito Mode) and then re-enter the web-store and re-attempt to make the purchase, to see if it works.
Should issue persist, could you clarify whether the error message you receive is something you get after entering your payment details and proceeding with the payment or does it pop up beforehand?
Let us know and we will take it from there!