as rlparker explained, you can't create subdomain without hosting top level domain in DH.
So you have to point your top level domain first. Remember to go to DH panel --> Domains --> Manage Domains --> Add New Domain/Sub-domain to add the top level domain after you point the DNS to DH.
Once the top level domain is hosted in DH, you can create as many subdomains as you like.
If you do not want to host the top domain in DH, you can follow rplarker's suggestion to simply delete top domain from DH.
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