If Havard's suggestion doesn't help, you might check that Rtools can be found. Your path should contain entries pointing to ...\RTools\bin and ...\Rtools\mingw_64\bin (the actual paths to these folders depends on where you decided to install Rtools). (The reason for my suggestion is that it looks from your screenshot that cp --- the unix/linux copy command --- can't be found, and I think this is supplied by RTools).
You can check this by starting a command session (DOS prompt) and then doing cp --help. If you get a help screen, then you know that cp is in the path.