How do I stop my own sub-account from running out of balance?

Each client sub-account has a wallet with you, just as you have a wallet with Votel.

When you create a new sub-account, there is a checkbox that marks it as your own. Checking it turns off billing for that sub-account, so its charges roll up to you and you do not have to watch its balance.

For a sub-account created without that option, open the sub-account's Billing tab and add funds to its wallet manually. The money goes to your own Stripe account, so you are not really paying anyone. Once the balance is high enough, the low-balance banner disappears when you log in as that account.