Yes it is possible, although the claim will be more complex and its chances of success will depend on what other evidence you can provide. Bank transfers, emails, WhatsApp messages where the debt is acknowledged, eyewitnesses, invoices issued and not objected to, can serve as proof of the existence of the debt even without a formal written contract. However, the less documentation there is, the more risk there is that the claim will not succeed if the debtor flatly denies the origin of the debt.