This is a bug in the builder script if the project path contains spaces. Already reported + fixed in
in 1c5bffe7879697cd7274a51f3035cb2d9c8929c4.
Since a new stable version is not released yet, you have to point the platform to the at least fixed commit or later, i.e., use the platformio.ini
[env:blackpill_f411ce]
platform = https://github.com/platformio/platform-ststm32.git#237956571a140374dada64008d63628788ab6e3b
board = blackpill_f411ce
framework = arduino
board_build.core = stm32
The alternative is to comment out the wrong line in
C:\Users\<user>\.platformio\packages\framework-arduinoststm32\tools\platformio\platformio-build.py
