An aggregate may not appear in the WHERE

create procedure DeleteProductID
@productid int = NOTNULL,
@forceDelete int,
@posCount int
as
select * from Sales.OrderDetails
if @forceDelete = 1
	delete from Sales.OrderDetails where orderid = @productid
else if @forceDelete = 0
	delete from Sales.OrderDetails where count(productid) < @posCount
	go

Msg 147, Level 15, State 1, Procedure DeleteProductID, Line 11 [Batch Start Line 0]
An aggregate may not appear in the WHERE clause unless it is in a subquery contained in a HAVING clause or a select list, and the column being aggregated is an outer reference.

declare @CountOrderId int
select @CountOrderId = count(productid)
from Sales.OrderDetails
group by productid

create procedure DeleteProductID
@productid int = NOTNULL,
@forceDelete int,
@posCount int,
as
select * from Sales.OrderDetails
if @forceDelete = 1
	delete from Sales.OrderDetails where orderid = @productid
else if @forceDelete = 0
	delete from Sales.OrderDetails where @CountOrderId < @posCount