If you are logged in as an admin and you still can't edit Memberships, there are a few things to check depending on what the exact issue is.
No Membership Menu Option
Make sure that the admin user has ‘remove_users' functionality. This is the capability that MemberPress uses to identify administrators. You can use a plugin like Members to view and update user role permissions.
Membership Edit Page is Blank
If you are on a version of MemberPress that is before 1.9.40 you can either update MemberPress, or make sure that the admin user is not subscribed to any memberships.
No Membership Meta Boxs are Visible
On the Membership, edit screen clicks on Screen Options in the upper right-hand corner and ensure that the checkboxes next to the meta boxes are all checked. This will need to be done for each admin user individually.