While testing an upgrade today, I was reminded of a requirement when upgrading to SharePoint 2016 using the database attach method – you must upgrade all site collections to the SharePoint 2013 experience before you attempt to attach the database to a SharePoint 2016 farm.
My test today is using a SharePoint 2010 content database and attempting an upgrade to SharePoint 2016. I first performed a database attach upgrade with a SharePoint 2013 farm, then performed the Test-SPContentDatabase PowerShell command on the database from my SharePoint 2016 farm.
It is important to notice that the LegacySiteDetected error shown above is an UpgradeBlocking error, meaning the upgrade will fail if you proceed. Following the direction listed in the Remedy section will fix the issue, but it does require working in the SharePoint 2013 farm to upgrade the site's experience to 2013. More information on how to perform that operation can be found here: Expand Upgrade a site collection to SharePoint 2013 [TechNet]