For SSRS expressions you need to use IsNothing
for NULL
checking, something like:
=IIF(
IsNothing(Sum(Fields!policy_id.Value, "DataSet1"))
, 0
, Count(Sum(Fields!policy_id.Value, "DataSet1"))
)
In fact the whole expression seems a bit odd; what are you specifically trying to achieve with your expression? Are you just trying to count non-null values?
=Sum(IIf(IsNothing(Fields!policy_id.Value), 1, 0), "DataSet1")
Also, your error seems to be saying that a parameter is referencing a field when this isn't allowed, which may not be solved by changing syntax; I think more information about what you're trying to achieve is required here.