Your calculation is incorrect.
First try is to include a + signal before IFNULL.
(IF ISNULL( "AE and Related CM") THEN "AE Preferred Terms" ELSE "AE and Related CM" END)
Hope it helps.
If having NULL in "AE Preferred Terms" means that there is no row in your data source for that combination, you're not going to address this with an IFNULL or ISNULL function.
You need to look at "data densification". You can google "tableau densify" to get all sorts of discussions about it. But let me point you to this link as a good place for discussion about this: