This is either because the invoice was already created, time/expenses have not been entered yet, or the client only has an outstanding balance.
First, click on Invoicing from your blue menu bar and click into Find Invoice. Pull down the Client drop down to choose this client and see if the invoice you are looking for is already created.
If there are no invoices then check the Work History tab under your project. If there is nothing listed then go ahead and create your work entries, then once finished an invoice can be created.
If you've already invoiced the client and they have an outstanding balance with no unbilled activity then click on Invoicing from your blue menu bar and click Create Statements. From there choose the client in the list and click to generate the statement to show the client their outstanding balance as of that day.