As of now, there is no way to transfer from Coinbase to Robinhood. This is because Robinhood only supports trading, and not storage, of cryptocurrencies.
So if you want to transfer your cryptocurrencies from Coinbase to Robinhood, you would need to first sell your cryptocurrencies on Coinbase, and then buy them again on Robinhood.
However, this may change in the future. As Robinhood starts to support more cryptocurrencies, it is possible that they will also start to support transfers from other wallets.
So if you want to be able to transfer from Coinbase to Robinhood, you should keep an eye on developments at Robinhood.