As of now, you cannot buy Diem directly on Coinbase. However, there are a few indirect ways you can do so.
First, you can buy Bitcoin or Ethereum on Coinbase and then transfer your cryptocurrency to an exchange that offers Diem trading. Kraken and Binance are two such exchanges.
Second, you can use a service like Changelly or Shapeshift to convert your Bitcoin or Ethereum into Diem.
Third, you can use a decentralized exchange like Uniswap to trade Ethereum for Diem.
Ultimately, whether or not you can buy Diem on Coinbase depends on how you want to obtain the cryptocurrency. There are a few different methods, but each has its own set of pros and cons.