This is a very misleading error message. What it simply means is that you currently have a “special page” open in Chrome (e. g. settings page, start tab or extension page). On these pages extensions do not work (no extension can work there).
Solution: just open a normal web page in the browser
You say you managed to import a test script you created earlier into Ubuntu: was the script created and tested in another OS environment? Are you running Ubuntu in Docker or a VM?
I don’t think Kantu likes anything that may interrupt a connection and I’m just wondering if there’s something that may be “getting in the way”.