Allow button in Privacy & Security System Settings does not appear

The allow button in Privacy & Security System Settings appear for your approval after you open Karabiner-Elements for the first time.

However, this allow button may not appear in some environments.

Reason

The exact cause of the issue has not been determined, but it may be caused by the corrupted system cache by a macOS issue.

This issue may be resolved by starting macOS in safe mode to refresh the system cache.

Restart your Mac in safe mode by this instruction.
Open Karabiner-Elements from Launchpad to ensure that Allow button is shown on System Settings.
Open System Settings and confirm the Allow button is shown.
If the Allow button is hidden, close System Settings and open it again.
Click the Allow button.
Restart your Mac and boot in normal mode.

Karabiner-Elements does not work in safe mode. You have to restart macOS in normal mode to use Karabiner-Elements.