Since they can make bookings and you can accept, I assume you have a subscription for Square Appointments and that it hasn't expired.
Can you provide more information about the specific steps or actions in the process that aren't working for you?
To add on a service, you'll need to select an already created appointment and scroll down in the details screen to the Services section, where Add Another Service should be listed.
One thing that has tripped me up is the creation of conflicting events or days off that I've forgotten about. Perhaps it's not letting you make appointments because they would conflict with something you've put in a Google calendar you've synced? Just an idea.
