If you get an error "Python is not detected" in a FreeNAS plugin you may follow this steps for fix it. In my case i was getting error from qBittorrent Web UI (#webui) plugin in FreeNAS when i was wanna install "search plugins" for qBittorrent.
1- Click jails and then click 3 dots near of plugin and click shell
2- Type in shell:
cd /usr/local/bin/
ln -s python3.6 python3
ln -s python3 python
ls -l | grep python
and check can you see "python -> python3" on screen
3- Go to iocage/jails/qbittorrent/root/etc folder and open rc.local with a text editor
4- Add export PATH="$PATH:/usr/local/bin"
5- Restart plugin
6- Open qbittorent webui. type "admin" in username input field, "adminadmin" password input field
7- Click search and then search plugins
8- Check for updates after u should see search plugins
9- it took my days to find solution. if you want, you can buy a coffee to me. Thanks