boot - System can not reboot after completing installation

Recently, I made a bootable USB stick with version 18.04 and
install OS from the USB stick to the hard disk.
The installation process follows the normal steps without any special configuration.

The whole installation process is correct and completed.
Then it shows a button to request doing the system reboot.
The steps of the reboot as follows:

1. Push the button, the system is trying to reboot.


2. The message show "Please remove the installation medium, then reboot".


3. Remove USB stick physically.


4. Type ENTER key and wait for a while.
   However, there is no response and message stops requesting for removing medium.

How about "then reboot"? Does it mean to press the ENTER key and wait for the reboot by itself, or push system reset button (send reset signal) physically?