Can ACH transfers be made to or from a prepaid card?

This depends. In most cases, the answer is no. A prepaid card can be of so many types. For example, a Netflix prepaid card or a Starbucks prepaid card. Such cards do not allow you to ACH funds into them, as they are sold over-the-counter with the card's value as the purchase price.

The idea behind the prepaid card is that they are not tied into any bank account at the time of issuance. However, prepaid VISA/Mastercards, which are sold over-the-counter, can be ACH’d into once the card is attached to the issuing bank account that it comes with (meaning you would have to enhance KYC and essentially be opening a bank account with the card issuer). In which case, ACH into the card would be possible.