You want to sort your tablix.
Sorting means you need to tell the desginer on what column(s) he needs to sort.
You expression will either return true or false, which isn't the expected result for a sorting expression.
Maybe you need something like
= Iif(Fields!Code.Value = "E" OR Fields!Code.Value = "Q", Fields!Code.Value, Fields!Blabla.Value)
how do you want to sort?