Summary: You need to get a free API key for the fixer.io service and provide it to the plugin.
The Bitcoin and Altcoin Wallets free WordPress plugin lets you use a number of online service providers to compute exchange rates between fiat currencies and cryptocurrencies. To translate between fiat and crypto, the fixer.io service is used. Release 3.4.0 of the plugin addresses some recent changes to the fixer API and brings some more improvements to the exchange rates system.
Be advised that unless you do this, fiat amounts will not be displayed correctly on the frontend. Additionally, some extensions such as the payment gateways require correct exchange rates to function properly. So get your key today!
https://gist.github.com/alex-georgiou/492196184f206002c864225180ca8fbb
If you disable an exchange rate provider, its data is now retained on the DB, but is no longer updated. Data is updated for all the exchange rates that you have selected in the checkbox list. For performance reasons, do not enable all the providers, only enable the ones you need.