404 page on the tag pages

Hello,

We have your plugin installed on our site and I wanted to know if it is possible not to display a 404 page on the tag pages which are private and for our
subscribers.
It should be possible to display instead a page with a login form to identify yourself as a subscriber.

Thank you

I've no idea what a tag page is.

Access denied pages tend to be entirely controlled by their extensions and what they do is up to them. Some send to a login form and others just throw an error. You'll need to discuss that with the developer of the extension you're using.