Scroll bar is not visible in a page of the Service Portal
One of the containers in the page has the "Move to Header" option checked.
Uncheck the "Move to Header" option
The "Move to Header" option makes a container on a Service Portal page act as a subheader. For example, if you have the homepage search widget at the top of a page and you want this to be visible at all times on this page so that if the user scrolls down, the homepage search widget is still shown at the top, you would put the widget in its own container at the top and check the "Move to Header" option.