Hello @ssalvi and welcome to the community.
As you are using Normal mode, you didn't tell if you already established a CAN bus where at least two nodes (your L4 MCU is among them + a CAN transceiver) are connected to it. Did you do that? if not, you cannot use CAN as UART or SPI to use it in standalone as it is or connect externally CAN_Tx to CAN_Rx. You need to use one of the test modes example the LoopBack mode in that case.
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