Hi Tortilla and welcome to the NS Community.

It looks like you cannot order a subscription because you do not pass the credit check. You can contact Focum to see why this is with the phone number in the screenshot, or filling out the online form on their website. 
It's also always possible to contact our Customer Service to see if you have any open invoices with us.

I must say that I found the text saying there is a good alternative, that there are subscriptions on balance with which you can get the advantages/discounts of the various subscription very misleading. There is currently only one subscription on balance that gives a discounts and that is Dal Voordeel op saldo, Apart from that you could get a monthly ticket Traject Vrij or Altijd Vrij. But the other subscriptions are not available if you can't get NS Flex. For many people that is not a good alternative.