I had this problem but I think I have just found the cause. The comment at the bottom of the window says "To edit a shortcut, click the row and hold down the new keys ..". In fact you have to click the right hand end of the row where it shows the existing shortcut or Disabled. The text there then changes to New Shortcut and the keys can be pressed.
I had this problem but I think I have just found the cause. The comment at the bottom of the window says "To edit a shortcut, click the row and hold down the new keys ..". In fact you have to click the right hand end of the row where it shows the existing shortcut or Disabled. The text there then changes to New Shortcut and the keys can be pressed.