When trying to retrieve license keys from Plesk panel, are you getting an error :
Licensing Server Unreachable: Unable to connect with licensing server. Please make sure that your network allows communication to ka.parallels.com:5224.
Login to your server via SSH and check if connections are getting through to the Plesk license server :
# telnet ka.parallels.com 5224 Trying 220.127.116.11...
If it is not getting connected, allow the IP ‘18.104.22.168’ in the server firewall.
# csf -a 22.214.171.124 Adding 126.96.36.199 to csf.allow and iptables ACCEPT...
# telnet ka.parallels.com 5224 Trying 188.8.131.52... Connected to ka.parallels.com. Escape character is '^]'. Connection closed by foreign host.
Once this is done, you can try retrieving the license keys from Plesk.