To reproduce:

Part design:
Create a body - create a sketch - XY plane.
Create a rectangle, close sketch and pad.

Select the end of the rectangle pad, sketch on face.

Bring in geometry, Add a rectangle to the side, pad.

Select pad, linear pattern, select reference, select edge of 1st pad. Nothing happens.

This works ok in freecad 0.20, but not in daily or stable.