Hi,
Sir, Did you used correct server API key and Pass while adding it on WHMCS panel ?
Please allow the virtualizor ports from 4081 to 4085 in the firewall on the WHMCS server you can add it from below command.
iptables -I INPUT 1 -p tcp -m tcp --dport 4081:4085 -j ACCEPT (for iptables)
firewall-cmd --zone=public --permanent --add-port=4081-4085/tcp (for firewall) firewall-cmd --reload
Also, check the server username and password is correct or not, You can check it from Admin panel >> Configuration >> Server info.