At the moment, "Split Issue" is only available from right clicking an issue on the Kanban backlog, so if an issue is on the Kanban board, it can't be split and you either have to use Clone or change to the Backlog view to use it.

If the issue is already "In Progress" that means moving it back to Selected for Development first.

Please make Split Issue available in all views and from the "." shortcut menu.