How To Fix 'The package could not be installed' While Installing Elementor

You are receiving this error message because you are trying to install Elementor as a theme whereas it is a plugin.

You have to install it via "plugins" and not via "Appearance > theme" on your WordPress Dashboard:

Please note also that you have to install Elementor free as well so that Elementor Pro can work given that the latter is an extension of the former.