A block order is when one party (for example a money manager) buys a large position on a particular stock for several clients instead of making many individual trades. If one of these accounts does not have sufficient funds for its allocation of the block order then the whole block order is rejected. To avoid this scenario you need to do a pre-check of every allocation by making a request to POST /trade/v2/orders/precheck. If the planned allocation matches the funding requirements for all accounts the you should be able to place a successful block order.
More Resources