By default, clients can't create sub-accounts on their own. You can enable this for each client individually — for example, if it's part of a paid plan or available on request.
Go to Profiles → Clients.
In the Actions column, click the edit icon next to the client:

In the window that opens, scroll down to find the Can Add Subaccount checkbox and check it:

Click Save.
Once enabled, the client will see a Subaccounts page in their web panel where they can create and manage additional accounts for their team members.