Insert plugins

Is your site on WordPress.COM?

If so, you cannot use plugins at all.

Is there a way to insert a slideshow ?? in the Reddle theme on wordpress.com ??
Not a slideshow in the right sidebar, but a just working feature on the blog itself.

I’m sorry but these forums are for self-hosted WordPress sites only. Please try asking your question on https://en.forums.wordpress.com/

As esmi mentioned, these forums are for getting help with self-hosted www.remarpro.com installations, where you install WordPress on your own server.

Your site – https://evaalbrandswaard.wordpress.com/ – is hosted on WordPress.com, so you can get help in the forums there, at the link she provided above: https://en.forums.wordpress.com/ – or you can use the contact form here: https://en.support.wordpress.com/contact/

WordPress.com does have a slideshow feature that can be used in any theme: https://en.support.wordpress.com/slideshows/