I believe that Sets do not evaluate CURRENTMEMBER in the query context so it's a bit like trying to sum to everything in a dimension.

Just do the following:

MEMBER [Measures].[Sales LTM] AS

SUM(

{[Calendar].[Calendar Month].CURRENTMEMBER.LAG(12): [Calendar].[Calendar Month].CURRENTMEMBER} //This is a little simpler than using ParallelPeriod

,[Measures].[Store Sales])