By default the Raspbian Lite ships with SSH disabled; this means the PI can only be accessed with a monitor and keyboard connected.
However, enabling SSH is straightforward. Log into your Raspberry PI and type the following command to launch the Raspberry PI Configuration:
Select Interfacing Options, scroll down and select SSH. Choose Yes to enable SSH and then click OK.
Choose Finish to close the configuration tool. The Raspberry Pi will now be available for SSH access.