You are converting a set to a member which you can't do

Try

member [Measures].[VOID] As

(strToMember("[CHB Date].[Year - Month - Period - Date].[Date].&[" + Format(dateadd("d",now(),-1), "yyyy-MM-T00:00:00") +"]") , [Chargebacks].[CHB_Tree].[CHB Type].&[IRS VOID] , [Measures].[Purchase - converted to USD])

Mack