The Forecast and Safety Stock together form the Order Point value shown in the header of an items detail view.
Since lead time is zero, and if the safety stock option uses lead time as the basis for the safety stock calculation, this in turn causes the forecast over the lead time and thus the order point to be 0 even if a forecast is being calculated as shown in the requirement plan table.
To learn more about order point, please review the article Basic Order Quantity and Replenishment Logic
Please note that a suggested order may still be created even if the order point and/or lead time is zero. This is because if the item is set as stocked and if there is forecasted demand a future order will be created once the forecast consumes the on hand.
Using the example above, if there are 6 units on hand with a weekly forecast of 2, at the beginning of week 4 a suggested order is triggered since the projected on hand becomes 0. The trigger in this case is 0 since there is no safety stock.
The order point sets the appropriate level of inventory to carry on hand and on order over the lead time to meet forecasted demand over that lead time and not stock out. The lead time is the time it takes to order and in turn receive the replenishment order.