Switched the Inventory Menu Appear Type to 'On Input Key', then setup the key in the Input Manager.
Result: Inventory menu doesn't appear when I hit the key during gameplay and I get the console warning message "No ' ' button exists - please define one in the Input Manager."
Have tried registering several different keys in the Input Manager with the same result.
Comments
Have you specified which key exactly that the menu should respond to? The Toggle key box should appear beneath the Appear type popup. You need to enter the name of the Input Axis you've defined.