To prevent Clients from going into a negative balance, navigate to System > Settings > Accounting > Client Balances and enable the Prevent negative balances setting.
Prevent negative balances setting, highlighted.
What the setting does
The setting behaves differently depending on who is logging the Lesson:
Tutors are blocked. If completing the Lesson would take the Client's Available Balance below zero, the Tutor sees a warning explaining the shortfall and cannot save the Lesson.
Administrators are warned, not blocked. Administrators see the same warning but can confirm and complete the Lesson anyway.
Note: the check runs against the Client's Available Balance, not their Invoice Balance. See here for the difference between the two.
Low Balance notifications
If you prefer not to restrict Lesson completion, you can instead enable notifications from the same page (System > Settings > Accounting > Client Balances). There are two, and they work independently:
Low Balance Notification — tick this and set a Low Balance threshold. TutorCruncher emails the Client when their balance drops below the threshold.
Low Balance Notification for manager — tick this and set a Low Balance threshold for manager. This notifies the Client's assigned Client Manager instead, so your team hears about it as well as the Client.
Note: Clients only receive the Low Balance email if they have Low Balance reminders switched on in their notification settings. See here for how to manage what your Clients are notified about.