As of October 2018, Mir is not on Coinbase. Coinbase is a digital asset exchange company founded in 2012. Mir is a cryptocurrency project that launched in 2015. The two have different purposes; Coinbase is meant for buying, selling, and storing digital assets, while Mir is focused on providing a payment system.
While Coinbase has listed a number of different cryptocurrencies, it has not yet listed Mir. There are many reasons why a particular cryptocurrency might not be listed on an exchange, including lack of interest from the exchange or the project, lack of liquidity, or regulatory issues. It’s also possible that Coinbase will list Mir in the future; the company has been adding new assets at a rapid pace over the past year.