hm. i'm not sure what instructions you are following but when using Particle Workbench, you do not need to manually check out the Device OS source.
these may help:
https://docs.particle.io/tutorials/developer-tools/workbench/#local-build-and-flash
the key thing is to walk thru the steps here: https://support.particle.io/hc/en-us/articles/1260801313350#running-a-basic-workbench-functionality-check
...and if you are still having issues, share the nonsensitive output of the Particle: Audit Environment command here so folks can better help with additional troubleshooting.
also FYI: