How do I cancel?

I signed up some time ago. Never got this going properly and would like to cancel. Please help.

Cancel what?
You don’t appear to have a PlexPass subscription, so all you can do is erase your user account.
Here: https://plex.tv/users/edit
But be careful, once you do that you can’t re-sign up with the same email address for ~2 months.

If you have also another plex.tv user acoount, you can go here and cancel any subscriptions, should there be any.