Change the status of the purchase order from Fully Billed to Pending Bill again after cancelling the initial bill.

In NetSuite, you cannot bill a purchase order with an expense line item more than once. After billing the purchase order once, the status of the purchase order will automatically change to "Fully Billed" and the Bill button will not be available anymore. This is the same behavior even though the bill you created was cancelled. This issue is logged under Enhancement #158732.

As an alternate solution:

1. Edit the purchase order

2. On the Items tab > Expenses subtab > Remove the expense account you initially added. Upon removing, there will be a pop up message saying that you are deleting an expense line item that was already billed. Just click OK

3. Add the expense account again with the same details as the deleted one
 

4. Hit SaveAfter saving, the status of the purchase order will now be Pending Bill and the Bill button is available again.