That link should just take you to the 'Manage Domains' section of the panel. You should not see a request for payment if you already have a hosting plan on that account.
If you have already created the sub-domain, just select the sub-domain using the URL: dropdown box. WordPress will then be installed to that sub-domain. The parent domain will not be affected.
Not at all, we all have to start somewhere and these things can be difficult to grasp sometimes.
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