Seems pretty clear:
You cannot use an aggregate in an expression used for the group by list of a GROUP BY clause.
Maybe you're trying to get the functionality...

Is Form.ID an nvarchar column or a numeric? If it's a numeric, why are you using nvarchar for the parameter?
The values in Qty.primaryanswer and WC.primaryanswer include the braquets? Or...

You really don't want a multi statement table valued function. You want to use an inline table valued function. The difference would be huge in performance, over 200 times faster...

Even better, make sure that you only have numbers.

SELECT
  bestphone = CASE
      WHEN MemberHomePhoneNbr LIKE REPLICATE('[0-9]',10) THEN MemberHomePhoneNbr
      WHEN AlternatePhone LIKE...

I would take a shortcut for the tables and get the information that is already stored in SQL Server. Then I would just need to count the rows in the...

To simplify even more:

SELECT
  ActiveProd   = COUNT( CASE WHEN StatusID = 1 THEN ProdID END),
  InactiveProd  = COUNT( CASE WHEN StatusID = 2...

You need to be aware of the implications. Changing a stored procedure into a function might not work because functions are meant to return something (scalar value or table) while...