One of the category's in the drop down list get error message "Page not Found"

That does not seem to be a wordpress site.

Its wordpress 4.3.1 running twnety fourteen theme with a with membership pro plugin added

the link you provided is not a wordpress site.

Is the right URL.. https://harmantraining.uk/lessons/ ?

Ahhh that is a wordpress site. I see a category that gets the error, so how many posts do you have with that category?

Juggledad

55 post

Andrew
Yes it takes you to the home page

What happens if you set your perms links to the default

Jugglehead

Same error page

Have you tried disabling all your plugins to see if one is causing the issue?

jugglehead

Just disable each one in turn but still getting same error

Clive

Debugging 101 – disable ALL your plugins at the same time and see if the issue occurs

juggledad

Still the same error message

Clive

I assume this was working, what changed? Did you update something?

juggledad
It’s never worked
Clive