Special Stock field in Requirement Class is in Grey Mode

Dear Experts

I want to change field special stock in requirement class 011 through T-code OVZG. But it is Grey Mode .As per already posted discussions .

1. Special stock field flow from vov7 .In My Item category special stock is Q But in ovzg it is blank.

2 Requirement class is already assign with costing so first deassign and then check.

Please Suggest the correct.