Why can't I pay my invoice automatically even though I've added my bank account information?

It is possible you added your bank information in the Company settings tab, which is not the correct section for invoice payments. We instead use this section when we need to process a refund to your company. 

When setting up a payment method, you need to bank you account information on the Payment Methods tab. Once you add a payment method, we then begin the process of verifying your bank account information. This process takes about 1-2 business days.

See also: How to set up a payment method