ABP has not really a join in common terms, because there is no ack or confirmation, so you will almost always join with success (OTAA is different). It's a fire-and-forget protocol, and even sending messages normally is without confirmation , thus the node normally does not know if the packet has been received. However, the provided examples are set to have confirmation, thus they fail if no gateway, or gateway not able to send downlinks, or node not able to receive.
If if you change true with false in the following line:
modem.endPacket(true);
You will succeed also in sending.
Please remember that, if you connect to TTN, you have to comply with fair access policy that include not sending more than ten downlinks/day (that is, avoid confirmation!).