If the install button in the app is unresponsive it is likely that the Spitfire Audio App doesn't have permission to write content to the location you have selected.
One way to solve this is directing the download to a different location (e.g. your Desktop) to check if the download starts, this will confirm that the issue is linked to the location you were previously trying to use and you will now be able to download the library.
To check permissions on Mac: right click the folder on volume and select "Get Info" and you will see permissions are displayed at the bottom of the info pane, where you can edit them.
To check permissions on Windows: right-click the folder or volume and click “Properties” in the context menu. Switch to “Security” tab and click “Advanced”. In the “Permissions” tab, you can see the permissions held by users over a particular folder.
If you are still having issues after trying a different location or changing permissions please get in touch with us by clicking on the help or chat button on the bottom right of this page.