How can I maintain a common task list and use it for all the projects?

A common task list cannot be created in Zoho Invoice.

As a workaround,

• Create a project with the common task lists you wish to have.
• Under Tasks, click on Import tasks from existing projects whenever you create a new project and wish to have the same set of tasks. Select a project from the drop-down in the pop-up that follows and select Import.

• Now the duplicate project will contain all the tasks from the existing project.
• You can edit the project details accordingly.