When you have finished drawing the copper area, exit the copper area drawing mode (right click or ESC) then click on the "Rebuild Copper Area" button.

You will not get any copper area fill unless the netname of the copper area is that same as one of the nets in the schematic. In other words if the netname assigned to the copper area does not match any of the netnames in the schematic then you will not see any fill in the copper area.

If your PCB doesn't have the GND net, please set copper area keep island as YES, or change its net same with one of the PCB net.

Please note that although you can get a copper area to fill even if the netname assigned to it does not exist in the PCB by setting "Keep islands" to YES,  having a floating copper area is not recommended as it can cause problems with crosstalk

@andyfierman Thanks, that solved the same problem for me.