I had the same issue on CentOS 8. My LibreNMS user was missing /usr/local/bin in the PATH. I could solve the issue by doing the following
export PATH=/usr/local/bin:$PATH
# If you want to make it persistent:
echo "export PATH=/usr/local/bin:$PATH" >> ~/.bashrc
If you want to fix this in the web interface set the following in your PHP-FPM config
env[PATH] = /usr/local/bin:/usr/bin:/bin