Currently on account payment is not allowed for customers while creating pick up orders which is highly inconvenient. If the customer has a credit limit then it should allow cashier to decide whether to create the orders and put it on his account.
Work around is to override the amount to 0 and then creating order and at the time of pick up/invoicing put it on his account. Two problems here. First we are creating orders with 0 payment which can go against the company policy and second this process is complex and cashiers could make a mistake

Please consider to allow on account payment for future releases.