Thank you for your message and sorry for the late reply.
I’m very new to this so I don’t know much and I’m sure I’m making some silly mistakes.
I don’t have other page builders and I have tried deactivating and activating again the plugins but nothing.
The button was there at the beginning, I tried to use it but gave me an error message but I didn’t investigate much. I went back the day after, and it wasn’t there anymore.
I seem to be able to create pages with Elementor from the Dashboard but I wouldn’t know what to do with posts.
And forgive my ignorance but what URL? Of the editing page?
Basically, if I want to add or edit a new page or posts I can only do it with Gutenberg.
Thank you again for the support.