Hey there! Inside cPanel >> Domains, you'll enter the subdomain you want to create, and then the name of the directory. By default, the path is already started as /home/username, so anything you enter there will get appended to that. For example, if you just put "sub.domain.com" in the document root text field, it will create the document root as /home/username/sub.domain.com
Can you try that and see if that works?