Hello,
The CoinPayments adapter extension is available on this site at https://www.dashed-slug.net/bitcoin-altcoin-wallets-wordpress-plugin/coinpayments-adapter-extension/
There are instructions and a troubleshooting section on that same page. The main plugin also has an accompanying PDF document which you might like to have a look at.
The error you get is because you have not connected the built-in Bitcoin core adapter to a running wallet. Simply disable the bitcoin core adapter. The plugin will instruct you about this.
regards