I would like to transmit few values from STM32F303 to STM32F407 through CAN. But the CAN enters into sleep mode after few transmissions. Tried to use HAL_CAN_WakeUp(&hcan) function but in vain . Any leads would be appreciated.

[Ozone]

There is no "sleep mode" in CAN. This happens in other parts of your firmware.

CAN frames have a quite strict timing, you will not recover quickly enough from sleep mode to properly receive a frame.

If your application is not battery-powered, do not use any sleep mode.