Unlike,
The compiler printed the following message, so I knew that “PM” was already defined.
C:\Users\username\AppData\Local\Arduino15\packages\Seeeduino\tools\CMSIS-Atmel\1.2.1/CMSIS-Atmel/CMSIS/Device/ATMEL/samd21/include/samd21g18a.h:476:38: error: expected ‘)’ before ‘*’ token
#define PM ((Pm *)0x40000400UL) /**< \brief (PM) APB Base Address */