It sounds like for the tasks error issue, you’ve facing the same issue mentioned here:
A much shorter thread resolving this is here:
In a nutshell, somehow the "task.autoDetect": "off" in VSCode’s settings.json may have been set accidentally, probably when VSCode asked about a task slowing down your computer - not telling you that if you didn’t tell it to leave them turned on, that PlatformIO would stop working.
For the intelliSense ‘problem’/error, try adding the full sub-path to the start of your include statement… i.e.
#include <NCO\LUT_8bit.h>
or perhaps
#include <NCO/LUT_8bit.h>