The wording could be a bit confusing.
It means when you are wielding a weapon that is not 2 handed or when you are wielding a versatile weapon in one hand.
I'm not aware the later is supported by the game ? AFAICS the only way to wield a versatile weapon onehanded is to also use a shield.
Though likely picking up the dualwielder feat and using an offhand weapon also works.
