We suggest you check the following:
1) Verify that your payer has funds on file
Go to Customer information and you will see there are saved payment methods.
2) Check the token is enabled for autopay.
Go to Customer information and make sure autopay switch next to the payment method information is swiped right.
3) Source of token
A merchant cannot determine which payment method is eligible for Autopay, only the payer has control over that. In that case, the merchant would have to send a "request token" email to make sure the payer wants to enroll a payment method to Autopay. (Only with customer's consent -by clicking on this email and saving a fund- is when a payment method will be saved as eligible for Autopay).