The information in this article is out-of-date, and is only being retained for reference purposes.
Many tokens do not have an accurate conversion rate, and if you're trying to swap or buy a custom token, or less commonly swapped token, that may be the issue.
If you don't think that's the issue, we recommend reviewing the gas limit you are setting. At times, if the gas limit you set is too low, MetaMask will provide "No conversion rate available" as the error message. Please refer to this support article about how you can calculate your gas price.
https://ethgasstation.info/ is a great reference for verifying the gas fee up to date.
Increasing the transaction fee will also speed up your transaction.