Plesk comes with a trial license key which is automatically installed to the control panel. This license key allows to create 1 Client account, 1 Domain account and 1 mailbox. Therefore, to fully use Plesk as you need, you should obtain a license key from Parallels or one of its resellers and install it to the control panel.
Once you have ordered new license key and received it to your e-mail, you can install it to Plesk. For this, use the lic_install operation. This operation is also used for installing additional license keys that allow using Plesk additional features (add-ons) like Acronis True Image, Kaspersky Antivirus, and so on.