You have to enable also CAN1 clock.
As mentionned in reference manual RM0090, “Dual CAN” part ( page 1069) :
"- CAN1: Master bxCAN for managing the communication between a Slave
bxCAN and the 512-byte SRAM memory.
-CAN2: Slave bxCAN, with no direct access to the SRAM memory."
Hence when using CAN2 it's obligatory to enable the Clock of CAN1
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