I see. A check was introduced in 3.2 to only show the tab when the client has the backup function enabled. Because the admin is not a client, it is not working for the admin. I have made a bug report: https://git.ispconfig.org/ispconfig/ispconfig3/-/issues/5962
This should fix it, you can try it on your system: https://git.ispconfig.org/ispconfig/ispconfig3/-/merge_requests/1350/diffs