One situation in which competitive bidding cannot be used is one in which there is not enough competition for the bid. For example, if there is only one seller of a given product, or if the product that is needed is unique, a bid would not really be possible.
A second situation is the case of an emergency. If a business or a government agency needs a good or a service very quickly, a competitive bidding process would not be appropriate. This is because the process would take too long.
In both of these cases, the business or organization should negotiate a price with the vendor rather than calling for bids.