It is known that this bug appears for some users ([BUG] 'CC' is not recognized as an internal or external command · Issue #20671 · MarlinFirmware/Marlin · GitHub).
The Marlin devs (and me) are desperately trying to track down why this happens because it works on our computers.
Does the file C:\Users\Robert\.platformio\packages\toolchain-gccarmnoneeabi\bin\arm-none-eabi-gcc.exe exist?
Have you tried removing the .pio folder in the Marlin project folder and retry building?
Have you tried removing all folders C:\Users\Robert\.platformio\packages\toolchain-gccarmnoneeabi* (there might be multiple ones) and retry building?
If that fails, please do as described in Issues building with fresh marlin firmware and PlatformIO 2.2.1 - #25 by maxgerhardt but in the [env:STM32F103RC_btt] section instead of the mega2560 one.
Logs can be posted here in Markdown code format (tripple-backquote block).