I have "inherited" the responsibility for a webpage

First, lets separate theme from content. What you edit via the post or page editor is content. Theme files a .php files. You will be tempted to edit them via Appearance->Editor, but this is a bad idea. Download the theme files to your computer via FTP if you want to edit them.

What do you mean “there is nothing shown” when you go to edit a post?

Please provide a link to your site.

OK – this helps.
Here is a link to the site:https://www.teamtour.no/

I am trying to edit the pages in the dashboard. I have made some new pages, they are possible to edit, eg https://www.teamtour.no/wp-admin/post.php?post=607&action=edit – but https://www.teamtour.no/wp-admin/post.php?post=225&action=edit is an example on nothing to edit.

I can’t see the “back end” stuff on your site (Nor should you give me any passwords!)

Is this one of the pages with nothing to edit?

https://www.teamtour.no/gruppereiser/gruppereiseforesporsel/

Can you post a screenshot of what you see when you try to edit it?

I have made some screenshots – but I do not know how to post them – do I need a mail-address?

Please put your screenshots on imgur.com

Doner album called Teamtour

bjoe — URL for the screenshot?

I uploaded them on imgur.com and called the album Teamtour