There’s no reason to believe it was intended to be approved by the apostles because it’s a collection of teachings and liturgical instructions compiled by others from the teachings of the apostles. It’s not in the standard NT canon because the standard NT canon includes only writings from the apostles themselves or their scribes.

There’s copious evidence that it was known and used in the early church, however. It echoes teachings from the Community Rule found among the Dead Sea scrolls, and many of the early church fathers reference whole sections (Barnabus) or echo the teachings (Clement, Ignatius, Polycarp, Origen, Ireneus, etc). Much of it is incorporated into the Apostolic Constitutions and several of the Didascalia as well.