Macintosh or Windows?
See the note at the top of this table.
"Note: On Mac OS, some keyboard commands for interacting with the operating system conflict with keyboard commands for interacting with After Effects. Select Use System Shortcut Keys in the General preferences to override the After Effects keyboard command in some cases in which there’s a conflict with the Mac OS keyboard command."
"Note: (Mac OS) Shortcuts involving function keys F9-F12 may conflict with shortcuts used by the operating system. See Mac OS Help for instructions to reassign Dashboard & Expose shortcuts."
thx knew I missed something.