STM32F429 CANopen with or without transceiver
You need to understand the functionality of the CAN Transceiver.

It "transforms" the CAN Rx and Tx Line of the microcontrollers CAN Controller to CAN High and CAN Low signal which is a differential signal.

If you want to connect your project to other CAN components, you probably need a CAN transceiver. If you just want to play around with two evaluation boards, a CAN transceiver is not needed, but the Rx and Tx signal are not as EMC proof as CAN high and low so you can not reach the same transmission path length.

By the way transceiver or not has nothing to do with CANopen. CANopen is just a protocol.