The default for Jira is to allow time estimates in most ways, but not put the field you need on the create or edit screens. Go to the project configuration, and find the "screens", descend into them and add "original estimate" to them. You'll need to be an admin to do it.
But. Don't try to use this for velocity or Scrum. Jira Software won't handle it usefully. Estimation really needs to be done at a story level, not sub-task. (There's an essay answer on why this is the case with Jira, with a lot of whinging from me at both Atlassian and their Customers, which I won't bore you with)
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