final String AUTHORITY = "com.android.externalstorage.documents";
Uri roottree = DocumentsContract.buildTreeDocumentUri(AUTHORITY,"primary:");
Uri sourceuri = DocumentsContract.buildDocumentUriUsingTree(roottree,DocumentsContract.geTreeDocumentId(roottree) + "Folder1");
Uri TargetUri = DocumentsContract.buildDocumentUriUsingTree(roottree,DocumentsContract.getTreeDocumentId(roottree) + "Folder2");
Uri resulturi = DocumentsContract.copyDocument(myContentResolver,sourceuri,TargetUri);
Copying Folder1 into Folder2 always return null. CreateDocument, DeleteDocument even MoveDocument working without any issue.