Run-Time Error 3075

Question

I have a program that control the inventory quantity in stocks. When the operator assign an inventory quantity to an order, the software should lock the quantity and doesn't allow other operators to assign the quantity.
I wrote a code and control it several times but I don't know why it show the error. My code is:
The error is in this line:
curStockAssigned = Nz(DLookup("AssignedQty", "qryAssignedInventoryBalance", strWhere2), -1)

Private Sub Form_BeforeUpdate(Cancel As Integer)  
    'I want to check the inventory that we can assign to a order  
    'I use StockInHand and calculate the inventory balance that we can assign  
      
     If Not IsNull(Me.cboInventoryID) Then  
        Const message = "There is not enough inventory for assignment"  
        Dim curStockInHand As Currency  
        Dim curStockAssignable As Currency  
        Dim curStockAssigned As Currency  
        Dim strWhere As String  
        Dim strWhere2 As String  
          
    'Find the criteria for comparing balance![54337-error.jpg][1]  
          
    strWhere = "True"  
    If Not IsNull(Me.Parent.StockID) Then  
        strWhere = strWhere & " AND tblInventoryTransaction.StockID= " & Me.Parent.StockID  
        strWhere2 = strWhere2 & " AND tblInventoryPermission.StockID= " & Me.Parent.StockID  
        strWhere2 = strWhere2 & " AND tblInventoryPermission.Assigned= 0 "  
    End If  
    If Not IsNull(Me.cboInventoryID) Then  
        strWhere = strWhere & " AND tblInventory.InventoryID= " & Me.cboInventoryID  
        strWhere2 = strWhere2 & " AND tblInventory.InventoryID= " & Me.cboInventoryID  
    End If  
      
    'find the quantity in stock from query  
         curStockInHand = Nz(DLookup("[Balance]", "qryInventoryBalanceControl", strWhere), -1)  
    'Now find the quantity that we can assign, it is the diffrence between StockInHand and Stock Assignedin Permissions  
         curStockAssigned = Nz(DLookup("AssignedQty", "qryAssignedInventoryBalance", strWhere2), -1)  
           
           
        If curStockInHand And curStockAssigned <> -1 Then  
            curStockAssignable = curStockInHand - curStockAssigned  
        If Me.AssignedQty > curStockAssigned Then  
              MsgBox message, vbExclamation + vbMsgBoxRight + vbMsgBoxRtlReading, "Not Enough Inventory"  
              Cancel = True  
        End If  
    Else  
       MsgBox "This type of inventory is not in the customer stock", vbExclamation + vbMsgBoxRight + vbMsgBoxRtlReading, "Discrepancy"  
            Cancel = True  
    End If  
        End If  
End Sub