You cannot create a predecessor relationship between two tasks because you get a dependency loop error.
Dependency loops happen when you have two or more tasks that depend on each other to be completed.
For example, Task 2 is a predecessor to Task 1, which means you must complete Task 2 before you can start working on Task 1.
If you try to make Task 1 a predecessor to Task 2, you get a dependency loop error because you cannot start Task 1 until Task 2 has been completed, but task 2 cannot be started until Task 1 is finished.
Workfront does not allow you to create a predecessor relationship between tasks if it creates a dependency loop. In order to fix the dependency loop, you must reevaluate the predecessor relationship between the tasks listed in the error message.