The visibility of the "Show SLAs" toggle for Visual Task Boards is dependent on the Tasks that are associated with the VTB itself. If you do not see the toggle, it is likely that none of the Tasks have SLAs. Out of box, this is most easily seen with Private Tasks (vtb_task) which are the underlying task type for the cards you add to a VTB using the "+ Add Task" button at the bottom of each lane. These tasks do not have any SLAs configured on new instances. You will need to create new SLAs for these tasks in order to show the SLA toggle for the board.
Follow the ServiceNow Documentation for creating SLAs. Make sure to specify the table as 'Private Task [vtb_task]'