A DNS entry for already exists. You must remove this DNS entry from this server or all servers in the DNS cluster to

Hello, I have 2 domains on the same server and share the same IP Address (created in WHM): 1. maindomain.com 2. clientdomain.com What I'm trying to do is host an app in a subdomain on portal.maindomain.com and have portal.clientdomain.com mapped to portal.maindomain.com. I added an A record on the DNS manager of clientdomain.com: A, portal.clientdomain.com, TTL 3600, SERVERIP of portal.maindomain.com (which's the same for both) When i go to the cpanel of maindomain.com to add an addon domain for portal.clientdomain.com it gives me the error below: (XID 8t9n73) A DNS entry for "portal.viptech.io" already exists. You must remove this DNS entry from this server or all servers in the DNS cluster to proceed. I know that they're hosted on the same server, its our own VPS with whm installed, but I have no idea how to map these subdomains together when they're both on the same server with different cpanel accounts. P.S I mapped portal.godaddyHostedDomain.com to portal.maindomain.com in the same way and it worked. So I would appreciate any help in that regard. Thank you!!