Weird – I expected PlatformIO to fail equally on any project then, but it just failing on Marlin is interesting. No idea what’s causing the error though. I remember having seen this type of error, PlatformIO is being serached for in the project directory instead of the actually correct C:\Users\<user>\.platformio\penv folder:
- Unable to build sketch: \platformio.exe" does not exist - #20 by gnybom
- Build Fails after most recent Updates - #8 by ollie1400
Try one of these workarounds, or open an issue in Issues · platformio/platformio-vscode-ide · GitHub to get help from the developers.