In the Modern User Experience (UX) - If a task has actuals, but the ETC is down to 0 (either by time entry consuming the time or by marking the task as completed and then re-opening it), ETC cannot be added to it. The ETCs revert back to 0 as soon as you click out of it. To enable editing we have to:
Tasks without Actuals seems to work fine. (e.g. I can mark it complete, reset it to open and update ETC to some non-zero value)
Release: All Supported Releases
This is working as expected with the current design in Modern UX and it works the same in Classic at the task level.
If you go into Classic UX and add the ETC column on the Task List and update the ETC there ; this is the same as if you go into the Modern UX on the Task Details Fly-out and update the ETC.
If there are actuals on the task and no ETCs remaining, to add more ETCs, this would need to be done at the assignment level.
For additional functionality available starting in 15.9 see: Assign Tasks By Using the Details Panel on Tasks Module of the Clarity 15.9 Release notes.
As referenced in the 15.6.1 release notes: "As a project manager or scrum master, you can now manage task assignments and monitor estimate-to-complete (ETC) values for your team members directly inside a project". For more information on this change see New Task Assignments for Projects in New Task Assignments for Projects in the Clarity 15.6.1 Release notes.
The modern UX does not support assignment level ETC updates. You can create a CHANNEL on the Blueprint that exposes the Classic Task List for drilling into the Task Assignments.
The TASK LEVEL ETC that is entered on the Task Details Fly-out is spread over all assignments under the following rules (same as Classic):