Can I be sued for just missing 1 payment?
If I'm being sued for a breach of contract and the contract is not up. I just missed 1 payment and its supposed to be a late fee.
As soon as one party to a contract violates a material term of the contract, even for the very first or only time, the other party may sue to enforce the contract. Payment terms, including late fees, are material; someone may always, under the law, sue to recover missed payments, once the payment has not been made in full, within whatever time frame is provided for it.
Whether it is worth it for the other party to sue is a different matter--often, it is not. Catching up on the payment is often a good way to remove any invenctive to sue; indeed, depending on the contract and the circumstances, by catching up, it may remove the right to take action.