As of October 2nd 2018 you will no longer be able to add new credits to your Envato Market account, and you will no longer see an Add Credits option in the header or in the checkout.
You can still use your existing credits to purchase items on Envato Market. If you don’t have enough credits for your purchase, you will be able to use a combination of credits and a PayPal/Skrill transaction. Existing credits will still expire 12 months after date of purchase.You can read more here.