Labour Leadership challenge Rule couldn't be clearer.

Chapter 4, Clause II, rule 2 B ii. “Where there is no vacancy, {nominations may be sought by potential challengers} each year prior to the annual session of party conference. In this case nominations must be supported by 20% of the Commons members of the PLP” ..........

I dont think it can be any clearer .....{nominations may be sought by potential challengers} Corbyn is not a challenger.