How to put in attendance on an inset or holiday day will depend on whether the day off applies to the whole school or just to certain student groups.
The whole school is off
You'll need to add the holiday or inset day to your academic calendar to remove the registers for that day using these instructions: Adding a planned school closure
There won't be any registers created for that day, so you won't need to take attendance or input marks. This tells data collections such as the Census that attendance didn't need to be taken that day.
Only some students are off
Make sure there isn't an inset or holiday day logged in your calendar to enable registers to be created for today. You can follow these instructions if you can't see today's registers: Why don't we have registers today?
You can see how to input attendance for students here: Partial school closure