What is happening is that the extension needs the permission to write the license key activation file on your computer but for some reason it didn't work.
How to fix it?
First, check that the folder Fontself exists under this path:
On Mac: /Users/USERNAME/Library/Application Support/Fontself
On Windows: C:\Users\USERNAME\AppData\Roaming\Fontself
If not, you can create it manually or by copy/pasting and running the following command:
On Mac, from your Terminal app:
mkdir ~/Library/Application\ Support/Fontself
On Windows, from your CMD app:
mkdir %userprofile%\AppData\Roaming\Fontself
On Mac you can access it from the desktop by pressing Cmd + Shift + G in the Finder and pasting this: "~/Library/Application Support"
After creating the Fontself folder, restart Illustrator or Photoshop.
Enter your license key, activate the extension.
To check if it worked just restart Illustrator or Photoshop, your extension should be activated and should not ask for the license key.
This should fix your issue in 99% of cases, but if you still have a problem just mention that you did these steps when contacting us either through the live chat or at hello@fontself.com