When attempting to copy multiple site collections, Sharegate displays the following error message:
The URL '/' is not valid for a site collection. You can copy the source site individually and specify a valid URL.
The error occurs when there is a root site collection or a host-named site collection in your selection for a copy site collection operation. When Sharegate encounters these sites, it will try to merge them directly in the destination default URL. For example, let's say you have a source site with the following address 'http://mySharepoint/'. When you migrate, you choose the default URL '/sites/' to create all site collections under this path . Since the source site resides directly at the root of the server, Sharegate will try creating it directly at the root of the chosen destination. This will result in the address 'http://myDestinationSharePoint/sites/', which is incorrect since this is reserved by SharePoint. Sharegate will prompt an error since it cannot create the root site at the destination.
To avoid this error, migrate each root site collection or host-named site collection individually and specify a valid URL for your destination.