As
@T4tomo said, you show a cassette lockring/freehub tool and you need one for a freewheel.
These are often a b****** to shift (from the image this will be its first (and ?last) time. You need to ensure the tool doesn't slip out and will (as Phil says) very likely need a steel extension to however your applying unscrewing (normal direction) torque, or maybe
@T4tomo 's vice technique (care with direction but often you can't generate enough torque).
Consider whether removal is necessary.