The most likely thing is that someone has edited the administrator role to remove the moodle/role:manage capability (or whatever it is called) and the moodle/site:doanything capability. You may have to go into your database manually, and edit the mdl_role_capabilities table to put back the allow capability for moodle/site:doanything for the admin role in the site context.
Edit: Sorry, that is probably excessibly cryptic. I'm sure there is a page on Moodle Docs that explains this in more detail, but I could not find it just now.
Roles and permissions
need help: Admin can not edit roles.
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