This feature is available in all editions.
Acceptance tests (also called tests) that are associated with a story/backlog item have a Detail Estimate, but those associated with defects or testsets do not.
What you can do
Backlog Items and Defects
For Backlog Items and Defects, the Tracking Level setting determines whether or not an estimate is available on a task or test. If you are a System Admin, you can choose the tracking level you want for your stories and defects. You can adjust these setting in Administration > Configuration > System > Tracking Level. See Tracking Settings for additional information.
The ability to configure tracking levels is only available in Enterprise and Ultimate editions.
Default Tracking Levels
The default tracking level for a story is at the Task/Test level. This means that you can enter detail estimate, to do, and effort (done) on tasks and tests, and the values roll up to the story. The default tracking level for a defect is at the default level. This means that you can enter detail estimate, to do, and effort (done) on the defect, but not on individual tasks and tests.
TestSets do NOT have the same configuration capability. Currently, the tracking level for TestSets is at the TestSet level only, so you cannot enter detail estimate, to do, or effort (done) for tests in a TestSet. In our backlog for regression testing, we have a Backlog Item for adding that same configuration capability on TestSets. Please visit IdeaSpace and comment on related ideas.