Sorry, had this bookmarked but didn’t have any fresh ideas.
The fundamental problem is that you don’t appear to have a directory with envs/lsst-scipipe-4d7b902/bin in it in your PATH. This is supposed to be set by the source activate line in loadLSST.bash. If it’s not, then something is wrong with your conda installation or possibly your shell startup files (.bash_profile, .bash_login, .profile, .bashrc).
You mentioned a fresh install. What version of newinstall.sh (what URL) did you use, and what version of the stack did you install?