This is not a common issue but can happen if your computer's operating system cannot correctly identify Final Draft's installer package.
If this occurs, go to Macintosh HD > System > Library > CoreServices and double-click Installer. Navigate to where the Final Draft installer file is located (probably in your Downloads folder). click the Final Draft installer file and click Open. The installation should begin.
IMPORTANT: Live technical support (phone, chat, email) for Final Draft 8 will end on December 31, 2017. You will still be able to find information and solutions in this knowledge base, in the online troubleshooter and in the FD8 User Guide.