It is always important that the sites and groups in your environment have valid users assigned as Administrators, Owners, and Group members.
What is a valid user?
A valid user is any single user who exists within and is employed by your organization, and has an associated email address to manage their sites or groups from.
Usually valid users will have their names and emails properly displayed.
What is an invalid user?
Invalid users are Orphaned Users, System Accounts, External Users, Security Groups, Company Administrator groups, and SharePoint Service Administrator groups.
If you have the correct administrative access, you can easily see if your sites have invalid users as Owners or Administrators using Insights.
Otherwise you can run a Clean Orphaned Users action on your site/group to ensure that there are no Orphaned Users.
External Users will have green icons and be marked External User. You can keep External Users as Members, but we do not recommend that they be assigned as an Owners or Administrators.
Security groups, Company Administrator groups, and SharePoint Service Administrator groups will have a group icon displayed rather than a individual icon, and will be named with their group type. You can keep a security group, but it is always recommended to have an individual valid user assigned as well.
System accounts have purple icons, are named System Account, and do not have a username or email address associated to them. We do not recommend removing system accounts, but you should assign a valid user when a system account is the only one present.
If you are missing valid Owners, Administrators, or Members, they can be added easily in Quick Actions.