Casual requires a recent installation of Transient 0.9.0+ from either ELPA or MELPA. If your version of Emacs is ≤ 30 but also includes Transient as a built-in package, you will need to set the customizable variable package-install-upgrade-built-in to t to enable updating it via package.el. Set this variable and proceed with installing Casual. Alternately invoking package-install with a prefix (C-u) will temporarily coerce the update of dependent built-in packages (example: C-u M-x package-install casual).
If you already have the latest version of Magit installed (via non-GNU ELPA or MELPA), you can bypass the above instruction as Magit already includes the Transient package as a dependency.