An ABA routing number is the nine-digit number that the American Banking Association assigns to financial institutions, and they are required for ACH transfers. ACH transfers are only available to freelancers who are U.S. Persons (see here) and whose bank accounts have an ABA routing number. All others must select another payment method.
To find your bank's ABA routing number, look at the bottom of your check for a group of nine digits between two ⑆ characters. It is usually at the bottom left, followed by your account and check numbers. If you aren't sure which number to use, call or visit your bank and ask.