In order to post a Sales Invoice, sufficient inventory must exist on the date of the invoice and at the location indicated on the invoice.
Common Issues and Solutions
1.) The date the item was brought into inventory is after the date of the invoice.
The system checks to ensure sufficient inventory exists on the invoice date. Look at the Invoice Date on the Sales Invoice and compare that to the date the inventory was received (the Item Receipt date on the Item Receipt, or if you don't use Item Receipts, the Bill Date on the Bill for the PO)
2.) The location of the item on the invoice may not be the same as the one where the item was received.
Look at the location on the line of the item on the Sales Invoice to ensure that it is the same as the one on the line on the Item Receipt (the Bill, if Item Receipts aren't used)