Fontself requires the Photoshop Generator plugin to be activated. If it ain't, first ensure that "Enable Generator" is checked in Photoshop Preferences > Plug-ins?

Like this: 

If the extension keeps asking for Generator, try restarting Photoshop.
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If you have more than 1 version of Photoshop opened, try to close them all and only start one. Running multiple versions is not recommended if you want to use Fontself Maker.
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Also ensure that your firewall is not blocking the required processes to run Fontself:
​https://help.fontself.com/troubleshooting/extension-panel-is-empty
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Finally, if your OS is a 64-bit Windows, mind that the 32-bit version of Photoshop does not run as expected. Install Photoshop 64-bit instead.