This can occur when you are running an outdated version of Final Draft 8. The remedy is to download and install the current version, 18.104.22.168. It will overwrite your existing installation and once it's on board you should be able to open, activate and run Final Draft without further problems.
The free 22.214.171.124 installer is available HERE.
IMPORTANT: Live technical support (phone, chat, email) for Final Draft 8 has been discontinued. You will still be able to find information and solutions in this knowledge base and in the FD8 User Guide.