Time tracking isn't available yet on Next-Gen projects but is on the short-term roadmap. You can check out what's on the roadmap and what's recently shipped here: atlassian.com/roadmap
I'd also recommend checking out this Community article where one of our Product Managers discusses what's available in Next-Gen projects and what hasn't been shipped yet.
This is a good example of where NG just hasn't caught up yet. While you can "log" time spent and remaining time there isn't an "original estimate". Estimation is limited to story points at this time. As PVS mentioned this is coming in 2020 per roadmap.
You might consider using the time logging feature in the meantime by simply setting the Remaining Time when the issue is created and leaving the logged time empty.
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