Stack - Installing Theme Plugins
When you first install the theme you'll see a popup on your dashboard from the theme asking you to install a number of plugins.
Allow the theme to install these for you. The plugins which are installed are dependent of the theme, the images below are for illustrative purposes only.
TIP - You can install all plugins at the same time by checking the box at the top of the this list, see below for an example HOWEVER, its advisable to only install the Page Builder you wish to use only, as the demo data provided is in two separate package (one for Variant and another for VC to avoid duplication)
Once done (the process should take no more than a few minutes) You will see a confirmation notice similar to the image below.
Now you have the themes plugins installed, you can continue to the next stage of the setup and import the demo data without any issues.
UPDATE (Since the Release of WordPress 5.0+)
Since the release of WordPress 5.0+ which included the Gutenberg Editor - users wishing to use Variant Page Builder must also install the Classic Editor plugin. This is only a requirement if using Variant, those using WPBakery Page Builder can opt not to do this, but can do so anyway if you prefer the tried and tested editing experience provided by WPs traditional TinyMCE editor (the standard editor used for many years)