Cannot access Network Admin Panel

Hi all,

I’m currently in the process of taking control of a new client’s website, I’m trying to add an account for my agency and clear the other users which the client doesn’t want.
I’m currently working with them through the client’s administrator account, but when I go to add a new user, the only available option is to “add existing” and I think that’s because it’s a network/multisite.
When I go to /wp-admin/network/ it displays a “You don’t have the rights to access this page”.
As far as the client has told me, the person who should have the Super/network admin role currently is a user that has their role as “none”.

How can I proceed in getting super/network admin access for my client?

Thank you all in advance,
Michael.