@jerendeb
Hi,

Is it a repeat task? If yes,you need delete the task which you set repeat strategy.

There are four ways to delete a task:
1.Directly drag and drop a task into Trash .
2.Right click on the task and select "Delete" .
3.Use the "Delete" key on the keyboard to delete the task.
4.Click "Delete" icon on task detail page to delete the task.
http://faq.doit.im/web.html#40

Shall you need any help or have additional questions, please do not hesitate to contact me back.

Thank you for your support!

Best regards,
Doit.im Team