generate a fill

Please be more specific and give us detailed informations, also about your system.

What you're try to?

Does this happen only on one image or on all? 

Do you get an error message?

Check your internet connection.

Check if the appropriate layer is locked.

Please provide a copy of your 'System Info'. This can be get from the Photoshop menu item Help > System Info. There's a copy button in the System Info dialog. Press this button and paste the info into your next forum post.

You can also try to reset the preferences of Photoshop using the steps described here:

https://helpx.adobe.com/photoshop/using/preferences.html#reset-preferences

It's recommended to backup your settings before resetting the preferences.

See here:  https://helpx.adobe.com/photoshop/using/preferences.html#backup-photoshop-preferences