How To Resolve Nested Aggregate Function Error?

I used case function to bucket aggregate counts of an item and want to divide the sum total of all counts into the value for each bucket (looking to have each bucket shown as a % of the total). However, I am getting an error that I cannot nest aggregate functions, which I understand, but need some help finding an alternate solution to achieve my goal.

Error:

Aggregate functions cannot be nested: [COUNT("values".CASE_AGE_CATEGORY)] nested in [SUM(COUNT("values".CASE_AGE_CATEGORY))]

Code:

SELECT Case_Age_Category, COUNT(Case_Age_Category)/sum(count(Case_Age_Category)) as Volume
FROM
(
SELECT DISTINCT(c.CASE_ID),c.CLOSED_AT,
    CASE
        WHEN TIMEDIFF('HOUR', ASSET_CHECKED_IN_AT, STATUS_CHANGED_TO_COMPLETE_HERE_AT) >0
        AND TIMEDIFF('HOUR', ASSET_CHECKED_IN_AT, STATUS_CHANGED_TO_COMPLETE_HERE_AT) <24 then '0-24 HOURS'
    
        WHEN TIMEDIFF('HOUR', ASSET_CHECKED_IN_AT, STATUS_CHANGED_TO_COMPLETE_HERE_AT) >24
        AND TIMEDIFF('HOUR', ASSET_CHECKED_IN_AT, STATUS_CHANGED_TO_COMPLETE_HERE_AT) <48 then '24-48 HOURS'
    
        WHEN TIMEDIFF('HOUR', ASSET_CHECKED_IN_AT, STATUS_CHANGED_TO_COMPLETE_HERE_AT) >48
        AND TIMEDIFF('HOUR', ASSET_CHECKED_IN_AT, STATUS_CHANGED_TO_COMPLETE_HERE_AT) <72 then '48-72 HOURS'
        
        WHEN TIMEDIFF('HOUR', ASSET_CHECKED_IN_AT, STATUS_CHANGED_TO_COMPLETE_HERE_AT) >72
        AND TIMEDIFF('HOUR', ASSET_CHECKED_IN_AT, STATUS_CHANGED_TO_COMPLETE_HERE_AT) <96 then '72-96 HOURS'
        
        WHEN TIMEDIFF('HOUR', ASSET_CHECKED_IN_AT, STATUS_CHANGED_TO_COMPLETE_HERE_AT) >96
        AND TIMEDIFF('HOUR', ASSET_CHECKED_IN_AT, STATUS_CHANGED_TO_COMPLETE_HERE_AT) <120 then '96-120 HOURS'
        
        else '>5 DAYS'
    End as Case_Age_Category
FROM TEST_DB.STAGING.DW_DECISIV_CASES c inner join DB.Seed.DEALER_MAPPING d on c.DEALER_ID = d.DECISIVDEALERID
WHERE d.DIVISION = 'K' 
    and RO_NUMBER is not NULL 
    and (d.DEALERCATEGORY ILIKE 'DEALER' OR d.DEALERCATEGORY ILIKE 'RTC') 
    and d.DEALERUSAGE ILIKE 'PRODUCTION' 
    and d.OWNERGROUPCODE !='S040'    
)

WHERE CLOSED_AT >= '2021-01-01 00:00:00.000'
GROUP BY Case_Age_Category
ORDER BY Case_Age_Category ASC

Screenshot of current output:

looking to have each bucket shown as a % of the total

Combining COUNT(...) with windowed SUM() OVER() to get the total across all groups:

SELECT Case_Age_Category, 
      DIV0(COUNT(Case_Age_Category), SUM(COUNT(Case_Age_Category)) OVER()) as Volume
FROM
(
  -- ...
) sub
WHERE CLOSED_AT >= '2021-01-01 00:00:00.000'
GROUP BY Case_Age_Category
ORDER BY Case_Age_Category ASC

db<>fiddle demo