When r10k install is called outside of the environment directory containing Puppetfile.simp, r10k cannot find the Puppetfile.simp file to be loaded by the Puppetfile. Both files were that was generated with by simp puppetfile generate.
This problem occured both with simp-vendored-r10k 2.6.2 and 3.1.1.
If we don't want to plumb the bowls of r10k, we need to use the fully qualified path to Puppetfile.simp. And the best way to figure out the path may be to force the user to specify the targeted environment when they generate the skeleton. Not super user friendly WRT to control repos (will need to fix the path if a branch is made from another branch in the control repo), but would be a workaround for now.