You can set up your 1-to-1 settings so that employees can book 1-to-1s themselves. If they aren't able to book 1-to-1s, this is because this setting isn't selected.
Click your name in the top right, then click Settings.
On the settings menu click Performance, then click 1-to-1.
Select Allow employees to schedule 1-to-1 with direct manager.
Scroll down and click Save.
An employee can now book a 1-to-1 with their manager.