subdomain.example.com can not be resolved.
# nslookup subdomain.example.com
** server can't find subdomain.example.com: NXDOMAIN
DNS service is not enabled for subdomain.
By default, Plesk does not create separate DNS zones for subdomains.
To fix the issue make sure to enable DNS service for subdomain in Plesk:
Home > Subscriptions >
example.com > Websites & Domains >
subdomain.example.com > DNS Settings by clicking Switch on the DNS Service
Please check the official documentation for your reference.