Why are clients getting "Your appointment is not booked yet. Please enter your name."?

If clients receive the error message "Your appointment is not booked yet. Please enter your name.", despite having entered their name when booking online, the cause is likely as follows:

Your account is set to not allow clients to pay in full, and the system essentially considers paying with a gift certificate to qualify as paying in full.

To fix the problem, you would need to change the "When you only require a deposit or card on file, should clients be offered the option of paying in full?" setting at the very bottom of Setup > Client Payments to "Yes...". Please note however that in addition to being able to enter a gift certificate voucher, clients will be able to pay in full.

The error message this produces --"Your appointment is not booked yet. Please enter your name." -- is confusing, as it does not accurately describe the problem, which we've let the development team know about.