Select runcommand from the RetroPie menu (or through configuration/tools in RetroPie-Setup). Ensure Launch menu is enabled. Check Launch menu joystick control. If joystick control is disabled, either enable it, or use a keyboard plugged into any USB port, instead of the gamepad, to access the launch menu.
Please do not post a support request without first reading and following the advice in https://retropie.org.uk/forum/topic/3/read-this-first