Welcome to Flyertalk.

This is a restriction which applies only to newly linked bank accounts, eventually you will be able to pay more often. This procedure protects American Express against a certain type of fraud in which a cardholder would use someone else's bank account to make the payment.

The workaround is simply to use your bank's online billpay service to push a payment to American Express. It's faster, easier and less expensive than sending a paper check and it will be accepted without any problem.