core_cm0plus.h: No such file or directory #include <core_cm0plus.h>
I used some AI to solve this problem. It will tell you to update to latest IDE and then delete the SAMD21 board packages and reinstall etc… but that does not help. You need to go to 
You need to download the .zip and extract that and find these 4 files:
cmsis_compiler.h
cmsis_gcc.h
cmsis_version.h
core_cm0plus.h
Then copy those files to:
C:\Users\yourName>\AppData\Local\Arduino15\packages\SparkFun\hardware\samd\1.8.13\cores\arduino
That fixed it for me.