Try this instead
=IIF(Fields!Records_Included.Value = 0, "", IIF(Fields!Interview_Completed.Value/Fields!Records_Included.Value = 0, "", Fields!Interview_Completed.Value/Fields!Records_Included.Value))
Or if you use this kind of calculation a lot, many of us do you could go to Report>>Report Properties and choose the Code tab and put something like this function in there:
Public Function DivideBy(ByVal Exp1, ByVal Exp2)
If Exp2 = 0 Then
DivideBy = ""
Else: DivideBy = Exp1/Exp2
Then in your SSRS expression you would put
and it would all work out just fine.
PS. I found this function code either on this site or elsewhere on the internet. I found it extremely useful and it is now part of my report template.