BIND : WARNING: key file (/etc/bind/rndc.key) exists, but using default configuration file (/etc/bind/rndc.conf)

Solutions:

1. Make sure that the key in /etc/bind/rndc.key is the same as in /etc/bind/rndc.conf
2. Copy rndc.conf key to rndc.key
3. Move /etc/bind/rndc.conf to any another location, for example /temp
4. Reload the rndc utility: #rndc reload

Source :
https://support.plesk.com/hc/en-us/articles/115000153813-Warnings-appear-in-syslog-key-file-exists-but-using-default-configuration-file