Simply, you cannot use the INSERT function inside a CTE. Assuming "Final" was one of the other CTE's in the multi CTE script, just move the INSERT INTO #Clients outside the CTE script. You seemingly don't need the temp table, since you are using CTE's, the Clients CTE will be available temp table or no. I suggest getting rid of the temp table altogether and continuing with the CTE method you already have in place. You may need to post more of the script to get better scope of the question.
,Clients as
(Select
Distinct
HospMastID
,HospCode
,ClientID
From
Final)