Here are a few reasons why an item cannot be allocated resulting in free stock not being updated and unable to post invoices:
Item is marked for dropship
Dropship items do not take stock from the warehouse but from when you receive items from the dropship supplier. For these types of items, you usually create a dropship PO from the SO document. It will be allocated if the PO is received. However, if you have stock in the warehouse available for dropship items, you can uncheck the Drop mark on the line item from the invoice or order to use the available stock. Always look for the Drop mark on the item if you are unable to allocate free stock.
Ignore stock levels on allocation is disabled
If the "Ignore stock levels on allocation" option from the Customer Preferences is disabled from the backend, it would cause the invoice documents to ignore the available stock (and not update the free stock quantity). If this was disabled to allow Auto Adjustment Stock from the POS, an alternative to this is enabling the back order option for the customer or the Default POS Shopper and it should allow you to create orders despite not having enough stock. The only setback of having it enabled is having negative free stock but could be avoided or relieved if you do a proper stock take or replenishment (e.g. creating POs or transferring stock from other warehouses). To post the invoices created when this option was disabled, check the "Ignore stock levels on allocation" and the "Allow to post invoice with no ship qty" options from the Customer > Setup > Preferences > Preferences (See Image 1). Note: If you have negative free stock, you may not disable this option again until this negative free stock has been filled. Post the invoices. Important: Updating of free stock will only take effect on new transactions after the preferences are enabled, so you would have to create Stock Adjustment - Out with the invoice document as reference to update the free stock with the new quantity. Consider keeping the preferences enabled for future transactions if you don't use the Auto Adjustment Stock from the POS and are now allowing backorders.
