You can’t just redefine compiler flags like that by writing them directly with no prefix in the platformio.ini. Use build_flags and build_unflags if you need to. In this case I don’t really see why you would need those flags, I suggest deletion.
Regarding the “Problems” section: I’m seeing two deprecation warnings regarding the usage of SPIFFS in your code. However, the warning can be safely ignored, it’s not a compiler error and won’t give you troubles in this version. Then there are two warnings in your library code.
The Adafruit neopixel warning is valid, but it has since been fixed in the main branch.
Meaning if you use https://github.com/adafruit/Adafruit_NeoPixel/archive/refs/heads/master.zip it should go away. Again, this is just a deprecation warning and does not affect installation.
The “Unable to open file” error is the more critical one. The path looks very long so you might be hitting the
MAX_PATH limitation. Either rename the project folder to shorten the path, or enable Windows long paths as general countermeasure.