What is the best ways to create a Task after that task is complete, but not yet create in teamgantt.

You can create a task after the time it was done, just like any other task. You can mark it complete right after you create it. That is helpful to document that you did that task.

I'm not sure I fully understand the question. But, to create a new task - simply move your mouse in the task list and the "add task" option will appear. Click that. Or, right-click on a task and you should see an option to add a task there. There are a lot of great tutorials and articles at teamgantt.com/support -- Hope this helps!