Symptoms
When attempting to copy a site collection or subsite, ShareGate Migrate displays one of the following error messages:
The default group '<GROUP>' will be ignored because '<USER>' is not a SharePoint group.
The default groups of this site will not be assigned because the "Custom Permissions" option is not checked.
Error code
AN-079-229
ST-363-229
Any error code ending with 229.
Details
From SharePoint 2007 and newer, a SharePoint site has the 3 following default groups:
Members
Owners
Visitors
These groups allow to easily manage user permissions by associating them to a specific group.
The error occurs because the destination user or group corresponding to the source group is not a valid value to assign. This may happen when mapping one of the default groups to a user.
Solution
There are 2 possible solutions to this issue:
Remove the problematic mapping in the Users and groups mappings and copy the site's custom permissions again.
Via a browser, go to the Set Up Groups for this Site page of the destination site and assign the desired groups to each default group. This page can be found by adding _layouts/permsetup.aspx to your site address. For example, a full URL would look like http://MyFarm/sites/MySite/_layouts/permsetup.aspx.
You should see a page similar to this:
From here, you can assign the desired existing groups or create new ones to use.