This question was answered by @hunky:

As per the normal rule, you need to add a card or send some money (USD 5) from Paypal to get the account activated, but I am not sure about their special rules (if they have any) for Students !!

Better wait for staff member to answer or if you dont get any reply here, you can open a support ticket from your account to seek clarification about the same..

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Hello, all

If you navigate to your billing page you should be able to see a link which says Apply Promo Code.

In order to activate your package you need to get your personal unique code and also to add a valid payment method to your DigitalOcean’s account (credit/debit card) or add some funds using PayPal.

Please note that you have taken advantage of a different promo code at some prior point each account is only allowed to use one promo code during it’s lifetime.

Hope that this helps!
Regards,
Alex