Welcome and thank you for choosing QuickBooks Self-Employed (QBSE), @vicstclair1.
The feature to add your clients or customers in QBSE is currently unavailable. However, the app will remember your client's information once you've added or entered it before when creating an invoice.
For more information about setting up your invoice preferences, creating, processing, printing, and exporting them as PDFs, and sending reminders for overdue ones, refer to this article: Create invoices in QuickBooks Self-Employed.