Many plugins don't work

Hello,

i have a problem: A friend asked me, if I can build his homepage - he had installed the Divi Theme. I installed a few plugins but then i recognized, that some plugins aren't compatible with Divi ("DSGVO Tool"). I installed also WPbakery - when i want to work with WPBakery in the Backend of the Website, i can't do anything on a single site and when i want to work with WPbakery in the frontend of the website, the plugin load eternally long and nothing happens. Then I disable WPbakery and installed "Elementor" Plugin. I could not see the Elementor in the frontend to work on the site. In the backend, I've clicked on the button "edit with elementor" but nothing happens. I don't know why this things are happening. I work with this plugins every day at my workplace with another theme and there, the plugins work perfectly.
Could this errors appear, because my friend installed the theme in an incorrect way (i don't know how he installed the theme)? Or why are this errors happen?

Thank you in advance (and sorry for my bad english^^)