PlatformIO on VSCode always displays Intellisense error, told me to update the includePath. Since includePath is project dependent, I suppose I should do update includePath everytime I make a new project? Building project is fine. I’ve also tried basic troubleshooting: reopening the program and even reinstalling it. Googled this many times as well. Couldn’t find anything useful. Should I just leave it like that? How can I remove that error message?
I do realize the additional text PlatformIO has added on the beginning of the c_cpp_properties.json file that says "!!! WARNING !!!": "PLEASE DO NOT MODIFY THIS FILE! USE https://docs.platformio.org/page/projectconf/section_env_build.html#build-flags"
. This redirects me to edit platformio.ini without knowing what to do.

Does Rebuild IntelliSense Index
(on the PIO alien head panel) do anything about it? You might need to do a build after running that to make it fully refresh stuff.
If it’s still a path issue, it would probably be a -I<include-directory>
build_flag
that you would need to add, but I think there is something seriously wrong going on there if the Arduino.h file isn’t automagically discovered!
oh I did rebuild the index! like I said, building (compiling) the project is fine. yeah I think I need to try to add that build flag. but even without it, I can successfully compile. so that’s not the problem, is it?
It’s not a problem if you can compile and upload… i.e. do stuff… without any issues. It may just be a configuration glitch, or maybe related to the configuration in your platformio.ini file.