If you get the error:

C++14 standard requested but CXX14 is not defined

You need to create some environment variables that R can use to compile the code.

If it doesn't exist create the directory ~/.R by performing:

mkdir ~/.R

inside this directory create the text file Makevars.

In your preferred text editor add the following environment variables to ~/.R/Makevars

CXX14 = g++

CXX14FLAGS = -O3

CXX14FLAGS += -fPIC

After this restart R and the error should disappear.