If the corresponding button is no longer visible, the "Log out" page has been deleted for the respective employee.
You can restore the page in the following few steps:
Navigate to Pages & Menu in the backend.
Click Create a site here.
In the Page Title field, enter "Logout," for example.
Under the Page Type item, select "07 Logout".
Make the page visible to all groups and set "Published" for Status.
Click Save.
Please wait... it will take a second!