Using Conditional Aggregation in SubQuery - sql

Question

Thanks to the help of another user, I was able to use Conditional Aggregation to get the data point I need. I now need to implement this into an existing query in order to get an SLA % for a date range (rather than each package). Previous post for reference: Pull a DATEDIFF between Rows with Distinct value and WHERE Clause
The below query was used when the assumption that the 2 timestamps in 'PackageTable' were accurate enough to calculate SLA. Since I found out they were not, I have to run the query on a different table (PackageTable_Audit) that basically records events in a row when a package moves from LifeCycleStatusId = 1 (creation) to LifeCycleStatusId = 3 (Assigned) to LifeCycleStatusId = 5 (Completed). As such, the SLA adherence % is the amount of packages that were completed in X seconds / total packages. Since I can't use a simple DATEDIFF in a sub-query, and thus have to use the aggregate function to get a DATEDIFF between rows, I'm not sure how to work it into the query.
I've updated my old query with the Conditional Aggregate, but I get the following Error:
"Cannot perform an aggregate function on an expression containing an aggregate or a subquery."
Query:
-- VARIABLE DECLARATION AND INITIALIZATION
DECLARE #StartDate varchar(10);
DECLARE #EndDate varchar(10);
SET #StartDate = '2019-06-01';
SET #EndDate = '2019-06-31';
-- TABLE DECLARATION ##################################################
DECLARE #TABLE1 TABLE("No. Packages in SLA" INT, "Total Packages" INT, "SLA %" FLOAT)
--#####################################################################
-- WHAT GETS INSERTED INTO TABLE 1
INSERT INTO #TABLE1
SELECT
A.NUM, A.DENOM, CAST(A.NUM AS FLOAT)/A.DENOM*100
FROM
(
-- COLUMN SELECTION. TWO NUMBERS WILL REPRESENT A NUM AND A DENOM
SELECT
(SELECT SUM(CASE
WHEN
datediff(second, MAX(CASE WHEN LifeCycleStatusId = 2 THEN rowDateModified END),
MAX(CASE WHEN LifeCycleSTatusId = 5 THEN rowDateModified END)
) < 172800
THEN 1
ELSE 0
END) AS IN_SLA
FROM PackageTable WITH (nolock)
WHERE lifecyclestatusid = 5
AND rowDateCreated BETWEEN #StartDate AND #EndDate)
AS NUM,
(SELECT COUNT(PackageGuid) As No_Packages
FROM PackageTable WITH (nolock)
WHERE lifecyclestatusid = 5
AND rowDateCreated BETWEEN #StartDate AND #EndDate)
AS DENOM
) A
SELECT "No. Packages in SLA", "Total Packages", "SLA %"
FROM #TABLE1