This question is in reference to Atlassian Documentation: Managing users
can an administrator on jira server log in as another user ? I don't see this option on the server documentation, but is available on the cloud documentation.
as you mentioned the cloud version of Jira has the possiblity to impersonate a user (see https://confluence.atlassian.com/cloud/log-in-as-another-user-776994835.html).
For server version you could use "Fast User Switcher (FUS) for Confluence". It also tracks usage statistics to comply with GDRP.
If you have Jira Cloud none of these plugins work. But it is already possible in Jira Cloud to switch user, as suggested by me. Follow the instructions in the link to make it work. If this does not work you don't probably have the permissions.
See also on top of the linked page:
"If their account is a managed account, only an organization admin can make updates to their profile."
If you have Jira Server, the difference between the "User Switchet for Jira" and Fast User Switcher (FUS) for Confluence is,
If you have any further questions don't hesitate.