This feature is available in all editions
If I track effort at the backlog level, do I have to enter a Detail Estimate for the backlog item?
Only a title is required by default. You can make other fields required as needed. However, Detail Estimates drive trend reporting and auto-populate the To Do value on the Add/Edit pages. This allows you to track real progress against what you originally thought it would take to do the work. Because values change as team members learn about the work they are doing, without a Detailed Estimate you'd be adrift without direction in the workstream.
Tracking level determines where the Detail Estimate, Effort, and To Do values are entered. If you set Tracking Level to be at the Backlog level, you would typically enter all three values (if you are tracking effort as well) for backlog items. If setting values at the Backlog level is not working for you, consider changing the Tracking levels.