As of right now, you cannot buy DFI directly on Coinbase. However, there are a few indirect ways that you can do so.
The first way is to buy another cryptocurrency that is listed on Coinbase, and then use that cryptocurrency to buy DFI on a different exchange. For example, you could buy ETH on Coinbase and then use that ETH to buy DFI on Binance.
The second way is to first buy USD or EUR on Coinbase, and then use that fiat currency to buy DFI on an exchange that supports fiat-to-crypto trading pairs. For example, you could buy DFI on Kraken or Gemini using USD that you bought on Coinbase.
Both of these methods are a bit roundabout, but they are the only ways to indirectly buy DFI on Coinbase at the moment. Hopefully in the future Coinbase will add direct support for DFi so that buying it will be even easier!.