@tig said:

Your selected face is at z=0... AND it's facing downwards [top face is looking down] !
It's a limitation that a lone face at z=0 will fail...
So try moving it up from z=0 [even 1mm will do!] ... [AND you can also try Reversing it using the context-menu].
It should then work OK...

WOW this works for me thanks a lot