If you have no special only locally modified platforms / packages installed in PlatformIO, please do a clean reinstall.
Delete your entire C:\Users\<user>\.platformio (or wherever your previous installation of PlatformIO folder and restart VSCode. It will detect that the core is missing and reinstall it.
Also make sure that your user’s PATH environment variable doesn’t maybe still contain a path to the old installation, if you ever added that manually.
And lastly, if the project still doesn’t build, delete its hidden .vscode folder and do a Ctrl+Shift+P → Rebuild IntelliSense in VSCode to reinitialize those files.