I fill all the mandatory fields but nothing happens when I click the add button.

Please enter the mandatory fields, which are as below:

1. Task Code, 2. Task Name 3. Task Description, 4. Assign To and click on Add. then your Task will be created.

If you want to add Task to the Projects in which you are not team member but projects are visible in your Account, please go to (Master Data–>Advance Settings–>Preferences–>Project Setup) their please check(true) on Automatically Include Tasks in Project Team’ so that you can Add Tasks to that projects.