KeenTools Core can be downloaded from its dedicated page
First, it's always good to review the installation instructions from our guide.
Anyway, if you have installed something and it doesn't work it's better to remove it first and relaunch Blender. Then please make sure you're installing the add-on zip file on the Blender add-ons page and then you're installing the KeenTools Core library on the installed FaceBuilder settings page.
If the machine where you're going to install FaceBuilder is connected to the Internet, you need to download the add-on file only, and install it using the 'Install...' button on the Add-on page of Blender preferences window, then you'd need to install the core library using 'Install Online' button on the FaceBuilder settings pane.
If that machine is not connected to the Internet, you need to download both, the add-on file and KeenTools Core library. Then you can install the downloaded core library using 'Install From Disk' button on the FaceBuilder settings pane.
To update the add-on you need to reinstall it. Before installing a new Core Library you need to restart Blender. The recommended process would look like this: remove the old add-on, restart Blender, install a new version of add-on and then install the new Core Library. We'll address this hurdle in future releases!
You need to open the add-ons preferences pane, find FaceBuilder, click the "remove" button and relaunch Blender.
Due to Blender license limitations we have to ship our core library separately.
Luckily, our add-on is able to download the core library almost automatically, but if the machine where you want to install FaceBuilder for Blender is not connected to the Internet, then you need to download not only the add-on itself but also KeenTools Core.
Unfortunately, no. FaceBuilder works only with Blender 2.80 and newer.
You can use it without any restrictions and limitations for 15 days after the first launch. After that you'd need a license, which you can get on this page.