There are typically no publicly-available DigitalOcean promo codes for existing users and are almost impossible to find. Nearly all the codes you find online are for new users who have never created an account.
This month DigitalOcean offers free $100 in Credit to any new accounts. The $100 credit can be used to order any products for 30 days.
Besides the offer Free $50 for new DigitalOcean account is still active, here is another way to help you try Cloud VPS hosting on DigitalOcean without paying, you can even no need to have...
Here is another way to claim Free Credit on DigitalOcean which up to $20 value, so you can use VPS on DigitalOcean free for 4 months ($5 plan).