When navigating to the Settings >> Advanced>> Client Portal you may be directed if you have not yet added the client portal to your subscription. Selecting "Setup Now" will begin the process of adding that to your account.
You may also navigate to Settings>> Manage Licenses to view the available options.
Once you have selected a plan, you'll then need to select the green check box followed by 'Purchase' to complete the process. This will then redirect you to begin setting up your Client Portal. For more information, please navigate here: Enabling your Client Portal