If Else in Stored Procedure Sql Server

If there are no matching row/s then @ParLngId will be NULL not zero, so you need IF @ParLngId IS NULL.

You should also use SCOPE_IDENTITY() rather than @@IDENTITY.

Try this one -

CREATE PROCEDURE sp_ADD_USER_EXTRANET_CLIENT_INDEX_PHY
AS
BEGIN

    DECLARE @ParLngId INT
    SELECT TOP 1 @ParLngId = ParLngId
    FROM dbo.T_Param
    WHERE ParStrNom = 'Extranet Client'

    IF (@ParLngId = 0)
    BEGIN
        INSERT INTO dbo.T_Param
        VALUES ('PHY', 'Extranet Client', NULL, NULL, 'T', 0, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, 1, NULL, NULL, NULL)

        RETURN SCOPE_IDENTITY()
    END
    ELSE BEGIN

        RETURN @ParLngId 

    END

END

Try this with join query statements

CREATE PROCEDURE [dbo].[deleteItem]
@ItemId int = 0 
AS
 Begin
 DECLARE @cnt int;

SET NOCOUNT ON
SELECT @cnt =COUNT(ttm.Id) 
    from ItemTransaction itr INNER JOIN ItemUnitMeasurement ium 
        ON itr.Id = ium.ItemTransactionId  INNER JOIN ItemMaster im 
        ON itr.ItemId = im.Id INNER JOIN TransactionTypeMaster ttm 
        ON itr.TransactionTypeMasterId = ttm.Id 
        where im.Id = @ItemId

if(@cnt = 1)
    Begin
    DECLARE @transactionType varchar(255);
    DECLARE @mesurementAmount float;
    DECLARE @itemTransactionId int;
    DECLARE @itemUnitMeasurementId int;

        SELECT @transactionType = ttm.TransactionType,  @mesurementAmount = ium.Amount, @itemTransactionId = itr.Id, @itemUnitMeasurementId = ium.Id
        from ItemTransaction itr INNER JOIN ItemUnitMeasurement ium 
            ON itr.Id = ium.ItemTransactionId INNER JOIN TransactionTypeMaster ttm 
            ON itr.TransactionTypeMasterId = ttm.Id 
            where itr.ItemId = @ItemId  
        if(@transactionType = 'Close' and @mesurementAmount = 0)
            Begin
                delete from ItemUnitMeasurement where Id = @itemUnitMeasurementId;

            End
        else
            Begin
                delete from ItemTransaction where Id = @itemTransactionId;
            End
    End
else
 Begin
    delete from ItemMaster where Id = @ItemId;
 End
END

if not exists (select dist_id from tbl_stock where dist_id= @cust_id and item_id=@item_id)
    insert into tbl_stock(dist_id,item_id,qty)values(@cust_id, @item_id, @qty); 
else
    update tbl_stock set qty=(qty + @qty) where dist_id= @cust_id and item_id= @item_id;

You do not have to have the RETURN stament.

Have anther look at Using a Stored Procedure with Output Parameters

Also have another look at the OUT section in CREATE PROCEDURE

You can try below Procedure Sql:

Create Procedure sp_ADD_USER_EXTRANET_CLIENT_INDEX_PHY
(
@ParLngId int output
)
as
Begin
    -- Min will return only 1 value, if 'Extranet Client' is found
    -- IsNull will take care of 'Extranet Client' not found, returning 0 instead of Null
    -- But T_Param must be a Master Table with ParStrNom having a Unique Index, if so Min is not reqd at all
    -- But 'PHY', 'Extranet Client' suggests that Unique Key has 2 columns, not just ParStrNom

    SET @ParLngId = IsNull((Select Min (ParLngId) from T_Param where ParStrNom = 'Extranet Client'), 0);

    -- Nothing changed below

    if  (@ParLngId = 0)
        Begin
            Insert Into T_Param values ('PHY', 'Extranet Client', Null, Null, 'T', 0, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, 1, NULL, NULL, NULL)
            SET @ParLngId = @@IDENTITY
    End

    Return  @ParLngId
    End

Instead of writing:

Select top 1 ParLngId from T_Param where ParStrNom = 'Extranet Client'

Write:

Select top 1 ParLngId from T_Param where ParStrNom IN 'Extranet Client'

i.e. replace '=' sign by 'IN'