After activating StartNext, you will see this notice:
Click Begin installing plugins. You will be navigated to Install Required Plugins page.
Simply check all of them (or all of required plugins and some recommended plugins you like) and from the drop down select Install, then hit Apply.
When finishing, it should look like this:
Next, go to Plugins >> Installed Plugins check all of them and from the drop-down select Activate and hit Apply
Required plugins: WPBakery Page Builder, StartNext Toolkit, CMB2 and CMB2 Field Type: Font Awesome.
Recommended plugins: Contact Form 7, Newsletter, Contact Form 7 and One Click Demo Import.