It seems that Coinbase has recently enabled XRP, but unfortunately, only for Coinbase Pro users in New York:
Coinbase Pro announced on Twitter that Ripples XRP is now available on their platform. The announcement revealed that currently, it is available only for the New York user-base.
Source: https://www.cryptonewsz.com/coinbase-pro-new-york-now-supports-ripples-xrp/19386/So you would have to exchange XRP for BTC first (you can use Binance or any other popular exchange for that), and then transfer BTC to Coinbase for cashing in.