Yes use it all the time.

Well that's just the popup you get when you visit the site for the first time, like you get on any site about cookies
I guess that you usually use the site without the www in the address, and so you accepted the cookies on the other domain, i.e. https://imcdb.org/movie_84987-The-Bill.html

This is what I get when I click on that link. :-

https://ibb.co/r2z3JGx

On the site, when you are on a movie page, e.g. https://www.imcdb.org/movie_84987-The-Bill.html in this case, do you see a link "Add a vehicle" in the bar above the gallery?

Sorry Idon’t understand what you are asking.

I think one of my posts was just deleted without explanation, if that helps?

I meant for adding things directly on the site rather than posting on the forum

I understand you didn’t write the code, but amazed photos aren’t resized rather than saying they are too big.

I’m not being blocked as such, just seem to result in multiple posts which then say they can’t be deleted.

Yes, I’ve added new movies and just added cars to existing movies.

It is not me who makes the forum, so I can't fix anything on it.
If you need help for submitting entries to the site, I can assist. Which point is blocking you?
Adding a new movie which do not exist yet? Or adding a vehicle to an existing movie?

No the picture attachment should really be resized, if too big.

Think I’ve tried most ways of submission, but unsure if any have worked well.

Maybe a bug, the picture attachment on the forum is not the thing that works the best.
But for submitting new pictures, the best way to do it is directly creating the page on the site, no need to use the forum for that.