If you are selling on eBay:
Please make sure that your billing information is completed. eBay requires you to set up a billing method with them before they enable your seller account. In order to do so, you can access your eBay account and introduce your billing details. If you are not able to find this section, please contact eBay via their live chat and request them to send you the link to complete your billing details.
You can also access here.