COUNT field incorrect or syntax error

Another possibility, if you want to avoid supplying data multiple times (replace the datatype of @artid with the correct data type):

$sql = $pdo->prepare("DECLARE @artid int = :artid
    SELECT stockamount, stockname, stockbalance.stockid, SUM(ABS(reservationtransaction.stockquantity)) AS reservedamount FROM stockbalance
    JOIN stock ON stockbalance.stockid = stock.stockid
    LEFT JOIN reservationtransaction ON reservationtransaction.articleid = @artid
    WHERE stockbalance.articleid = @artid AND ((changeddate > DATEADD(yy,-1,GETDATE()) AND inventorydate > DATEADD(yy,-1,GETDATE())) OR stockbalance.stockamount <> 0)
    GROUP BY stockbalance.stockid");
$sql->bindValue(':artid', $productId);
$sql->execute();

This will only work in an RDBMS that supports DECLARE statements.

The number of parameters specified in SQLBindParameter was less than the number of parameters in the SQL statement contained in *StatementText. SQLBindParameter was called with ParameterValuePtr set to a null pointer, StrLen_or_IndPtr not set to SQL_NULL_DATA or SQL_DATA_AT_EXEC, and InputOutputType not set to SQL_PARAM_OUTPUT, so that the number of parameters specified in SQLBindParameter was greater than the number of parameters in the SQL statement contained in *StatementText. SQLExecute Function

placeholders must have unique names even if they have the same value

$sql = $pdo->prepare("SELECT stockamount, stockname, stockbalance.stockid, SUM(ABS(reservationtransaction.stockquantity)) AS reservedamount FROM stockbalance
JOIN stock ON stockbalance.stockid = stock.stockid
LEFT JOIN reservationtransaction ON reservationtransaction.articleid = :artid
WHERE stockbalance.articleid = :artid2 AND ((changeddate > DATEADD(yy,-1,GETDATE()) AND inventorydate > DATEADD(yy,-1,GETDATE())) OR stockbalance.stockamount <> 0)
GROUP BY stockbalance.stockid");
$sql->bindValue(':artid', $productId);
$sql->bindValue(':artid2', $productId);
$sql->execute();

All the columns that are not in any arithmetic function must go in the GROUP BY clause. see below:

SELECT stockamount, 
       stockname, 
       stockbalance.stockid, 
       Sum(Abs(reservationtransaction.stockquantity)) AS reservedamount 
FROM   stockbalance 
       INNER JOIN stock 
               ON stockbalance.stockid = stock.stockid 
       LEFT JOIN reservationtransaction 
              ON reservationtransaction.articleid = :artid 
WHERE  stockbalance.articleid = :artid 
       AND ( ( changeddate > Dateadd(yy, -1, Getdate()) 
               AND inventorydate > Dateadd(yy, -1, Getdate()) ) 
              OR stockbalance.stockamount <> 0 ) 
GROUP  BY stockamount, 
          stockname, 
          stockbalance.stockid