Content Aware Fill Dialog error message I do not understand

[Gdadtraveling]

Using Content Aware Fill Dialog and I can't select the "Custom" Sampling Area Option. I keep getting the pop-up message "Could not fill because there are not enough remaining pixels to fill the hole. The change cannot be applied." ?

[c.pfaffenbichler]

Could you please post screenshots with the pertinent Panels (Toolbar, Layers, Options Bar, …) visible?