Can I use CAN and USB at the same time with STM32F042K6U
Not in that package.

CAN be routed to PB8/PB9, but PB9 is not available in your package. You need to use one of the 48 pin packages to be able to do this.

If all you need is CAN receive, then your 28 pin package can do both CAN and USB, since PB8 is available as CAN_RX on pin 1 in that package.

I'm not a fan of the code generator, but the STM32CubeMX package has a decent pin planner that can help you play "what if" for a given part/package.