CLion can't find Arduino.h Header and other system headers

It’s no general problem as you can see in the following screenshot

image

if your file is located under src - as I assume - and not under test. If the later is true, there is an open issue as you can see here:

resp.
https://youtrack.jetbrains.com/issue/CPP-21173

Nevertheless you could try to re-init your project (located under Tools → PlatformIO → Re-Init)