As of now, you cannot buy Dero on Coinbase. The popular US-based cryptocurrency exchange does not list Dero (or any other coins) on its platform. This is likely because Coinbase only lists the most well-known and established cryptocurrencies. Dero, while a promising project, is still relatively new and unknown in the space.
That said, there are other exchanges that do list Dero (e.g. Binance, KuCoin, etc.). So if you’re interested in buying Dero, you’ll have to look elsewhere.