Hello,
Envato authority removed Envato Credit system
https://forums.envato.com/t/were-removing-envato-credits-from-market/195361/888
so you are not able to add credit on your account.
but you can purchase anything to using Skrill, Payoneer, Paypal Credit Card, Master card etc.
Still have any question you can contact with Envato Help
https://help.market.envato.com/hc/en-us/requests/new
Thanks