I’m traying to use reservation but the first issue regarding them was the inventory.

I created a Sales Order no. 5000 with qty 6, my inventory in item ledger entry was 5 and I had a purchase order with qty 2. I reserved 5 qty form item ledger entry and 1 qty from purchase order.

I created oneother Sales Order no. 5001 with qty 3. I do not reserved any thing. I posted the SO 5001 before SO no. 5000.

System do not prevent me to post.

So I’m not clear which is the meaning of nav reservation? Because i reserved the inventory on SO no. 5000

You may check your Item Card->Planning tab for this item …

If the Reserve option is set to Always, then disregard of your manual reservation, that SO that will be posted first will consume your inventory first.