It is important that your sites have valid users assigned as Administrators or Owners.
Using reporting, you can validate if there are any invalid users assigned in these roles. Using this information, you can then assign valid users if needed, or remove the invalid users from the assigned permission.
Not sure what ShareGate Migrate considers a valid user? See: Valid Users
Index
How-To
Run the Report
Select All reports.
Click Create custom report.
Select Site Collection as your Object Type.
Select Site Owners or Administrators under Filters.
From the drop-down, select Contains.
Type in System Account or Security Group
Note: To find accounts that contain both a System Account and Security Group, you can use the "+" icon to add an additional field. To find sites that contain either a System Account or a Security Group, you need to run two separate reports.
You can Continue without saving, but we recommend you Save & Continue if you want to Schedule your report to run at an interval that works for your business needs.
Select the target for your report.
Click Run now.
You will now see a list of sites that contain System Accounts or Security Groups as a site owner.
Take action On Invalid Administrators/Owners
Click to select the site(s) that only have System Account or a Security Group assigned as the Site Owner.
On the Quick Actions pane under Owners or Administrators, click + Add.
Select a valid user to add as Owner/Administrator.
If more than one site is selected: You will be brought to an options screen.
If one site is selected: You can add the Owner/Administrator directly from the Quick Actions Pane by typing the name and clicking Save.
Note: You do not need to remove the System Account or Security Group as Owner/Administrator as long as at least one other valid Owner/Administrator is assigned. If you would like to remove them, you can do so in bulk using the Remove Permissions option, or on one single site by clicking Remove under the Owner/Administrator sections in the Quick Actions pane.