Why can't I able to add an Action in Topic?

 The New button is not visible because the agent needs to be deactivated first before adding or removing actions. In the screenshot, it also shows the message: “To add or remove actions, your agent must be deactivated.” 

Try clicking Deactivate (if the agent is currently active), then go back to the topic’s Actions tab. You should then see the option to add a new action. 

Also make sure the action you want to add is already available/created in Agent Actions. Once the agent is inactive, it should allow you to assign it to the topic.  The Salesforce documentation on adding actions to topics may also be useful if you want to check the setup requirements.