Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/Microchip Technology/ATSAME51J19A/CAN0/TEST#0x0
TX=CORE
Test
Loop Back Mode
Control of Transmit Pin
0 (CORE): TX controlled by CAN core
1 (SAMPLE): TX monitoring sample point
2 (DOMINANT): Dominant (0) level at pin CAN_TX
3 (RECESSIVE): Recessive (1) level at pin CAN_TX
Receive Pin
https://github.com/cmsis-svd/cmsis-svd-data