Payment Method & Cash Account: In Acumatica, go to Configuration > Banking > Payment Methods. Confirm that the RAMP payment method is active and mapped to your Cash Account.
Vendor Setup: On the Vendors screen in Acumatica, verify that the Vendor (and any other vendors being paid) is active and properly configured.
Batch Payments in Ramp: In Ramp, make sure batch payments are disabled so that invoices sync individually. If batch payment is enabled, ensure "Pay by Line" is not enabled.
Test Payment: After making the necessary changes, retry the sync.