Hello @Mehwish Rao
Welcome to Atlassian Community!
Per the tags in your issues, I believe you are currently using Jira Cloud application, is it correct?
As you can see in the blog post below, the Legacy mode has been completely removed from Jira Cloud application on February 11, 2019:
As mentioned in that blog post, we've decided to remove the legacy mode because it was only enabled on about 1% of Jira Cloud sites, and were causing more problems for many of those sites than it solves.
Although the Original Time Estimate can not be restricted from editing, you can still track any changes performed to it in the history tab.
Let us know if you have any questions.
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