If I had to guess, this is caused by the difference between subdomains which are created when the account is created and those which are added later on (hence why the former are in the Addon Domains section and the latter are in the Subdomains sections). If so, that might just mean why you can create CNAME records for some subdomains and not for others.
I’ll need to do a bit more investigation to be sure that this is the case and see if that can be fixed.
But if you don’t want to wait for that, you may be able to work around this by removing the subdomain from this account and creating a new account for it instead.
@mmaklilu did you tried to select an subdomain when you added a new CNAME?
If YES: that’s why gave you that error.
If you’re trying to install an SSL certificate you need to follow these exemples:
If you’re using Let’s Encrypt:
On the SSL domain main page, you’ll see two CNAME records: one for _acme-challenge .example.com and one for _acme-challenge.www .example.com, each with their own Destination. These are the records you need to add.
If you’re using GoGetSSL:
Click the Request SSL Certificate button, or click the Pending certificate already there.
On this page, you’ll see one CNAME record, with a destination ending at comodoca.com . This is the CNAME record you need to add.
You should keep this browser tab open, because you’ll need these records later.
If not installing SSL certificate, you need to contact an Admin.