Make sure you press the correct keystone if that's true than make sure you your keystroke isn't conflict with other keystroke.

You can change the popups keystroke at addon preferences panel. On the other hand you can change the other keystroke that causing conflict with popups keystroke.

Popup will only show if the active object is a mesh object and this mesh object has at least 1 vertex group.

If the active mesh has 0 vertex group a little info will appear at bottom and says "Object has no vertex group"

Popup will not show if it's in Sculpt Mode or Particle Edit Mode.