"A customer is needed"
A sale that is unpaid or part-paid must be linked to a customer, so you always know who owes you. Pick an existing customer or add one on the spot. A fully paid sale can use Walk-in.
"Not enough stock" or "Only N left"
You are trying to sell more than you have. If two people sell at the same time, whoever saves second is told how many are really left. Adjust the quantity, or record a supply first.
"Selling price cannot be below cost"
The price you entered is below the product's cost price. Raise the price. The owner can allow below-cost sales in Business Settings, but those need the owner's approval when a member makes them.
The sale is "waiting for approval"
Sales recorded by Staff wait for a manager or owner. They count once approved.
An offline sale shows "Needs review" or "Needs a customer"
See *Offline POS: keep selling when the internet is down*.