The "Service unavailable" error when adding a bank account for United States (US) domestic withdrawals, means that you are either entering an ACH routing number instead of a wire routing number or you have reached the limit of adding two bank accounts.
Make sure that you are entering (Fedwire) account and routing numbers and not an ACH account and routing numbers when adding a US bank account.
You can contact support if you need to remove a domestic USD bank account from your Kraken account. You are only able to have two domestic USD accounts on profile for withdrawals.
Kraken does not currently support ACH bank transfers.