There will be occasions that you are not able to teach a specific lesson because the venue is unavailable or a teacher is on holiday, etc. In this case, you are able to make a single lesson inactive.
This is separate from holidays where you can add these to apply to all classes for time off. Find out how to do that here.

For this, go on 'Classes' > 'View All' and select the class where you want to make a lesson inactive.

Find the class that you need and click on it. Then head to the tab 'Lessons' to see all your lessons. If you want to make a lesson inactive, you have just to switch off the button under "Status" on a lesson to change it.

This means that this particular lesson will not appear on your calendar and if you have your class cost frequency set to 'lesson' or 'week' it will discount that lesson from your billing when you generate your invoices.