Cannot select multiple categories for listing

And of course how to I create a new clean database?

Or do these steps describe it better: https://pagely.com/blog/reset-wordpress-website-without-plugin/ although I have never done something like this before. It doesn´t seem something a newbie like I could do.

I think I solved it.

I now see the tab Custom Fields. I will set up again and add listings to see if it truly solved.

Thank you so much for your help.