Hello Fabio Benevento
The TxBusy can be caused because the Tx is not being listened. It could be helpful if you detail me the value of the CANx_ESR1 register.
Also, make sure that the loopback mode is active, the ID from Rx and Tx needs to match.
Let me know if this is helpful, if you have more questions do not hesitate to ask me.
Best regards,
Omar