build fails fastLm.o

First of all, you have the RcppEigen package installed, right? If so, my guess is that you need to have certain environment variables set that you presently do not have set. I specifically have these in mind:

CXX17 = g++
CXX17FLAGS = -g -O2 $(LTO)
CXX17PICFLAGS = -fpic
CXX17STD = -std=gnu++17

On my local machine (on which I have root privileges), I defined these variables in /usr/local/lib/R/etc/Makeconf . On my Institute's computing cluster (where I do not have root privileges), I defined these variables in ~/.R/Makevars .

Since you're on a Linux-powered system, I assume you're using gcc as the compiler.

Let me know if that helps.