Yes the file is there… The problem is 100 times more complicated…
The problem is with PIO, it magically tries to use absolute path to the project. The project is located at Z:\Project, but PIO tries to read C:\Project. This is a bug introduced in the past 1-2 month and only a bug, when the drive Z: is a subst drive that’s real location is on drive C:, like C:\DRIVE_Z. PIO uses only the original drive letter C:, but with the path on drive Z:, and certainly there is nothing.
After several days I figured out the root cause, it is reproducible, and I finally created an issue in PIO: Error: Unknown board ID - due to wrong absolute path used instead of relative path in case of subst-ed Windows drive · Issue #4155 · platformio/platformio-core · GitHub
If anybody here can add anything to that issue, please comment on the issue.