Hello,
Good to know that you fixed the problem.
The installation instructions already mention that you should enable the get_callback_address permission. See step 3 at https://www.dashed-slug.net/bitcoin-altcoin-wallets-wordpress-plugin/coinpayments-adapter-extension/
About the other error (JSON parse error), please see the troubleshooting guide in the documentation where it is covered extensively.
kind regards