This page: https://docs.moodle.org/25/en/Backup_of_user_data
talks about having to be a site admin or at least a manager.
There is a capability to control the backup of user data moodle/backup:userinfo, separate from the capability to backup courses. By default this capability is allowed for the manager role only.
I am a site admin and also have an account with the manager role and I still cannot backup user data. You can also check for who has that ability by running the Site Administration ->Reports -> Security overview Then look at the output under backup of user data.
In my case it shows that I can. But on my backup screen it shows a Red X and a gray lock. All this I discovered yesterday.
There is another post about this type of problem as well. https://moodle.org/mod/forum/discuss.php?d=314852