It seems you have R installed for both x32 and x64 architectures but you don't have RTools for x32.
If you don't need the 32-bit version I would recommend uninstalling it to avoid this kind of issues, but another way around is using the no-multiarch option.
install.packages("xfun", INSTALL_opts = '--no-multiarch')
Although, if I'm wrong and you are using the 32-bit version only, then you would need to check that you have a suitable version of RTools properly installed, or update your R version so you can have access to precompiled binaries.