I don’t speak french (please do, more people can understand you in this forum then) but from what I understand, it’s the same problem as already described in Can not compile: arm-none-eabi-g++: command not found - #8 by maxgerhardt. The external platform you’re using gives you the Linux version of the compiler, not the Windows one.
Go into the platformio.ini of the project and add
platform_packages = toolchain-gccarmnoneeabi@http://dl.platformio.org/packages/toolchain-gccarmnoneeabi-windows_x86-1.80201.190214.tar.gz
at the start of the environment under [env:LPC1768].