The master document is shared by a workgroup, and someone else is currently working on the subdocument.
Someone who previously worked on the subdocument set file sharing to the read-only option.
The subdocument is stored on a read-only file share.
Click the subdocument icon for the subdocument you want to
convert. (Click Master Document View on the Outlining toolbar if you cannot see the subdocument icon .)
On the Outlining toolbar, click Remove Subdocument.