Please look into this calculation
If window_min(Min([nthtransaction]))<=1 and [Article]="Pen" then 'show' else 'hide' End
Window_Min(Min([Nthtansaction]) finds the lowest nthtransaction at the level of Category for each Customer.
and set the calculation to compute using [Category] after placing on the Filters shelf and select to show values of "1"
Hope that helps