I'd be more than happy to help you invoice your client. Can you clarify if there are any obstacles or error messages you're receiving while trying to accomplish this? In the meantime, I'll provide you with general steps needed below:
Invoicing a client in QuickBooks Online
From the home screen, click the Plus icon (+) and select Invoice.
Select your client from the Customer (▼) dropdown at the top and enter an email address for them to receive the form at.
Fill out the rest of the form as necessary. Make sure you've selected a Product/Service that you're billing the customer for.
Once finished, select Save and send to email the invoice.