As Carmen Nunes suggested, you can use setup.
The requirement that you are asking for cannot be fulfilled unless you have created a sales Order and have added Lines to it.
Although you can restrict the Item from adding in the Sales Order by customization wherein you can check if Inventory(Quantity On Hand) <=0 and do not let that Inventory be added.
Regards
Jyotsna
jyotsnanav.blogspot.com
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