Hi,
Today we faced this issue while importing managed solution extracted from our Dev Environment to Test Environment (CRM 2016 Online Update 1).
As suggested by few of the posts online, the culprit was the System Customizer role. We removed it from our solution and then imported it back and it got imported successfully.
Helpful Post
http://crmkid.com/tag/cannot-add-a-root-component/
Hope it helps..