The button Delete task is in the Bottom toolbar. The Bottom toolbar is a panel with five buttons at the bottom of the Main application panel. Each button in the Bottom Toolbar gives you quick access to an action: create a task, create a section, set a date, move the task to another project, delete.
Method 1. Highlight the task and click on Delete task.
Method 2. Highlight the task and press Backspace
Method 3. Right-click on the task. In the drop-down menu select Delete task.