"To validate the debt" means to confirm or verify that the debt is legitimate and that the person or entity owed the money has the right to collect it. Full definition
Third, if the debt collection agency fails to validate the debt with sufficient evidence, they may not attempt to collect any money from you. (treesfullofmoney.com)
Here is the catch - they can validate the debt after that and normally they do. (thecreditguy.tv)
This can often be the main reason why debt collectors can't always validate a debt. (nomorecreditcards.com)