Can you have a payable account as a prepayment?

Generally speaking, no. A payable account is a liability account where the company owes something. If you have a payable on your books it's something you "owe" another person/company. Prepayment refers to something that is "prepaid" or paid before hand. Prepaid accounts are generally considered "assets" on your books. For example "Prepaid Insurance" would be listed in your assets.

If your company prepays for something, you list it on your books as an asset of some form. If you ordered computers from another company and prepay for receiving them, your books will list this transaction as an account receivable, which is an asset account because now the company you paid the computers for "owes you" and it will be an asset on your account.

Now if we reverse this transaction and a company pays your company for computers that it has not received yet, then it becomes a liability on your account (i.e. account payable) because you now owe that company something.

Therefore a payable account of any kind can not be listed as a prepayment.