what is Reserved Instance Monthly Prepayment ?

There are three types of reserved instance payments https://aws.amazon.com/ec2/pricing/reserved-instances/pricing/

All Upfront: pay for the entire Reserved Instance term with one upfront payment.

Partial Upfront: a low upfront payment and then a discounted hourly rate for the instance for the duration of the Reserved Instance term.

No Upfront:no upfront payment and a discounted hourly rate for the duration of the term.

If you believe that you are on a No Upfront Plan but that a charge has been levied as if you are on either All Upfront or Partial Upfront then this isn't something that can be resolved on re:Post. Please raise a Billing support case and AWS's billing support team can assist https://support.console.aws.amazon.com/support/home#/case/create