I try to generate Threats and Mitigations, but nothing happens.
Automatic Threat Generation requires to have some generation rules defined already in the system, and that the rules apply to the specific scenario.
How to address the problem
The first potential issue is that you do not have the Auto Threat Gen Extension Library. If so, install it from the Downloads page.
Verify if the Threat Model includes Threat Types and Mitigations. If not, then you have to import a Template supporting Auto Threat Generation rules.
If this does not solve the problem, you may need to check if the Threat Model has any generation rule defined, and then if those generation rules apply to the existing objects or not. To do so, you need to switch to Expert or Pioneer Execution Mode, if you are working in Simplified mode, and then to use the troubleshooting approach described in Auto Threat Generation rules definition.