Try IF ISNULL([Field]) or [Field] = 'NA/A' THEN 'Missing' ELSE [Field] END. Let me know if you have any questions.
Thanks Ivan! I did actually try that after the original post and it was still keeping the fields separate.
It works fine for me at combining fields. Maybe you have something misspelled in your formula.
You may have other values which aren't being explicitly picked up...
I note also between your question and Ivan's answer the N/A has changed... #N/A >> NA/A
In this instance I would suggest putting your 'field' on the row shelf (by itself, without filters) then you can see all values for the field.
You need to deal with each instance separately... null is 1, N#/A is 2, any others?
As Ivan mentions, the only issue can be spelling and/or your data (or filters?)
Given all the inputs, this method will work as you want.