A negative Available means you have promised more than you are holding. It is a backorder, and Brahmin Solutions lets it happen on purpose.
What the number is telling you
Available is what is left after everything already promised to customers:
Available = On Hand − Allocated
You hold 6 front lights. Confirmed orders account for 20.
Available reads −14 — you are 14 short of what you have promised.
That number is doing its job. It is telling you to buy 14 more before those customers expect them.
Why confirming is allowed to do this
Because refusing the order would be worse. A customer wants 40, you have 26 — you take the order, ship what you have, and send the rest when it lands. See Ship part of an order.
If confirming were blocked, you could not record the demand at all, and nothing would tell you to reorder.
What to do about it
Order more. The item will be showing on your reorder report if it has reorder points set. See Read the reorder report.
Ship what you can now. Committing takes whatever is available and leaves the rest outstanding.
Check nothing is stuck. Stock committed to an order that is not going ahead is still counted as promised. Uncommitting a dead shipment releases it — see Undo a shipping step.
When it is worth a second look
If Available is negative and you cannot find the demand behind it — no open orders that explain the number — that is worth reporting. Gather the item's SKU code, what On Hand and Allocated show, and any order numbers you have checked.