Are you using JIRA Cloud? If so, I think you'll need to find someone in your organisation who is in the 'site-admins' group.
A possible way to find out who this is, might be to use an advanced issue search with the following query:
creator in membersOf("site-admins")
This will show you all issues created by people in the 'site-admins' group. Hopefully it will return some issues. Then you can look at the reporter of one of those issues to identify someone in the group. This person should have the user management option.
Someone may know a better way, but this was the best I could quickly think of!
Hope it helps.
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