I have customers who defaulted on payment. Can I sue them?
Yes. You absolutely can sue them. Default on payment would constitute a breach of contract. A breach of contract means that you had an agreement with these customers. They materially breached your agreement. Generally speaking, non-payment would constitute a material breach of a contract. And you were damaged by that breach. Again, if you were due money and you didn’t receive that money, that would constitute damages under a breach of contract action.