If a supplier cannot fulfill an order (or an item on an order), then you will need to cancel the order (or an item on the order) in order to re-order that/those jobcard item(s) from an alternative supplier.
This can be done in the Browse Orders window under the Purchasing menu:
In this window, highlight the purchase order you would like to cancel, or if it is only a single item on the order, highlight that item in the Order Details list.
Then click the Cancel button (to cancel the entire order) or the Cancel button at the bottom (to delete a single item). This will allow the job detail item(s) to be ordered from an alternative supplier.
1. Cancelled the Wrong item on an order
If you cancelled an item and then realize that it was the wrong item you cancelled you can simply go to Uncancel it at the bottom of the Order details block as shown below.
Should you need to Uncancel an entire order, simply select the order you cancelled at the top and then click the Uncancel button. Each of that order's item will have remained cancelled at this point, so you will need to uncancel each order item that must be uncancelled using the Uncancel button for the order's items.
NB: Please note that you cannot Uncancel an item if you have already re-ordered it on another Order or if you GRV'd the cancelled item. If you need to uncancel an ordered item that has already been GRVed then you will need to Undo the GRV before uncancelling the order item.