I came across this same issue (and found this thread looking for a solution via Google-fu).
Reinstall Rtools (from https://cran.r-project.org/bin/windows/Rtools/).
As mentioned above, devtools was failing due to callr issues, which came from processx. I tried updating processx 3-4 times, but got a "WARNING: Rtools is required to build R packages but is not currently installed. Please download and install the appropriate version of Rtools before proceeding:" error.
Once I updated Rtools, everything worked fine.