I just tried with:
new Buffer(“sOmESecReT=”).toString()
and
new Buffer(“sOmESecReT=”).toString(“utf8”)
and
new Buffer(“sOmESecReT=”).toString(“base64”)
Still no luck, as for the user_id, thanks for pointing this out. At the moment, the user_id is mine as I am testing locally with my Twitch account 
I read the whole JWT library docs and can’t seem to find the reason why it’s not working 