Question about message received when student is attempting to place a request via CUNY+'s My Account

This afternoon I helped a new student try to place requests for two books. He went to the circulation desk to establish a record and came back to the reference desk. When we tried to place the requests for the books, the message "Borrower expiry date has been reached." I didn't know what it meant. The student went back to the circ desk but I was then helping someone else and then the next shift came on. Could someone please explain what the message means? Thanks.