A purchase requisition records an internal demand. It tells purchasing what the business needs, where it is needed and when it should arrive. It isn't a supplier invoice, and saving it doesn't increase stock or post an expense. A later purchase order can refer back to the requisition.
Use ME51N to create the request. Check material, quantity, plant and delivery date before saving. Zero and negative quantities don't describe a normal new demand in this exercise. An item requested in pieces must use a material whose base unit fits that meaning. The saved requisition number provides a stable reference for the buyer, even when several requests concern the same part.