I don't find any crypto currency with the name of Rinkeby eth in the coinmarket cap site,so did you spelled your crypto name correctly or you bought it from ICO sale? If a coin not existed in CMC means it don't have any value so you need to wait until it listed on exchange then only you can convert it.
For clarification,
Some altcoins/tokens aren't listed in Coinmarketcap.
Not all tokens that is not listed in Coinmarketcap doesn't have a value.
And lastly, even a coin is listed on exchange, it does mean that it should be listed on Coinmarketcap.
To OP: I think you already know what is the answer. But like the poster above, Im also curious where did you get it.