Problem with Permanent-link structure change

Hello,
at the beginning i have to say that i never did anything related to WordPress/Ubuntu before 1 Month. Now, i have the project for my work to set up our new shop with WordPress. I allready have set up our new dedicated server with Ubuntu 20.04 and all other needed components. I succesfully installed the newest version of WordPress and it was running without problems so far. Unfortunately i encountered the first problem today. I tried to change our permalink-strucutre from “Plain” to “Post name” and after this i get a “404” error everywhere, except on the homepage.

I thought that was an common problem, and i found many posts about this on the internet. I tried everyhthing(!) that i found in the last few hours, but nothing seems to work.
Here is what i did so far:

1) I checked the .htaccess File and all permissions are there

2) I checked the .htaccess file for the RewriteEngine, everything is correct there

3) I′ve checked the apache2.conf and AllowOverride=Yes/Require all=granted

4) I′ve restarted the apache2 service and the whole server

5) I′ve read that there should be AllowOverride=All too in the 000-default.conf file
at the sites-available folder but there is nothing in like that. I tried to add the
following there and restarted everything after this but that hasn′t changed anything

<Directory /var/www/>
Options Indexes FollowSymLinks
AllowOverride All
Require all granted
</Directory>

I don′t know what to do next, that was really everything i found in the past few hours of research.

Please apologize my bad english!

Thank you all so far,
please stay safe

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