Your Mac must already be using macOS High Sierra or later on an APFS-formatted startup disk. Find out which format your startup disk is using.

You don't need to reserve storage space for the new volume, but macOS needs as much as 20GB of available space to install, depending on the version of macOS. If you don't have enough space, the installer will let you know.

About This Mac includes a storage overview: Choose Apple () menu > About This Mac, then click Storage.