CAN0 confustion in Datasheet for ATSAMC21G uC
You can choose pin33 or pin37 as TX line and choose pin34 or pin38 as RX line.
If you look at page 675 (34.4 Signal Description), it says Refer to for details on the pin mapping for this peripheral. One signal can be mapped to one of several pins. You'll need to configure the correct pins by setting some bits in the registers. But a quick look at the "Related Links", shows that they explain it well in the datasheet.