Where can I locate the bank account routing number in my Business Central vendor checkbook?

Navigate to your vendor bank account details, go to the Transfer tab section, and review/enter the transit (routing) number.
 
Third-party link for more information.
https://stoneridgesoftware.com/eft-payments-in-dynamics-365-business-central-stop-relying-on-paper-checks/#:~:text=Navigate%20to%20your%20Bank%20Account,create%20a%20specific%20ACH%20Batch: