The domain "sub.domain.tld" may not be created by "USER" because "domain.tld" is already owned by another user

Symptoms

When attempting to add a subdomain of a domain owned by another user, I receive the following error:

CPANEL_ERROR: Error: (XID 7gqcxh) The domain “subdomain.domain.tld” may not be created by “USER” because “domain.tld” is already owned by another user.

Cause

This error occurs because, by default, cPanel users may not manage subdomains of domains owned by other users.

Resolution

1. Log into WHM.
2. Navigate to Home / Server Configuration / Tweak Settings
3. Search for the Tweak Setting named "Allow cPanel users to create subdomains across accounts to On."
4. Change this setting to "On".
5. Click "Save" at the bottom.