Contract validation error - You can't associate this project and task to this contract line

Summary:

Created a new contract, Added Line 1 and line 2, associated project and task to these lines. Getting below validation error.

You can't associate this project and task to this contract line.

A project and task in the same branch but at another level of the task hierarchy is already associated with a contract line. You must select a project and task from a different branch, or from the same level of the same branch of the task hierarchy.

Project WBS is as below.

Task 1 (associated with contract line 1)

Task 2 --> Task 2.1, task 2.2 (associated with contract line 2), task 2.3

Could you please let us know the exact validation procedure so that we can ensure what task can be associated to the contract