If you can't start an optimization you might have missed to set up all options. Please take care of messages in the infocenter at the very bottom of the Graphics Window!
You can find warning and error messages during the optimization in the log-file in the result tab.
Differentiated log files are saved in the project folder in your workspace (in the folder the optimization was started in). Further information here.
Possible mistakes for a non-running optimization:
No design space is selected - one object has to be selected
MSC Apex Generative Design 2019 works best with the binary stl file format. It can process the ASCII stl file format but there can be problems. Please try converting your stl file in a binary stl file and try again.
An stl file is missing in the project folder.