Why are the scroll bars not visible in the Gantt Chart and how to enable them?
According to Apple GUI recommendations scrollbars are invisible per default and appear in a window when there’s content that doesn’t fit into the window’s current size.
You may choose however overwrite this preference option. Just open the 'General' system preference and enable 'Show Always'.
Please find more information on how to use scrollbars in macOS on the Apple Support website.