You can assign a pass to an appointment any time, and if you assign this pass to the appointment, the occurence will be deducted from the pass.
Step-by-step guide:
Go to your Calendar.
Click on the “Create” button in the upper right corner or just click on the place in your Calendar where you want to add the event.
Select Booked appointment.
Fill the information of your client.
Click on Pass checkbox. If the client has multiple passes, you can chose one from the dropdown menu.
Save when you're done.
If you see "No passes available for this guest" under the Pass checkbox, the issue could be either one below:
Your client does not have valid pass for the service you chosen.
The client's pass does not have remaining sessions left.
Your client's pass is not valid.