You need to convert the parent folder to BrancObject
if it isn't one yet:
// The source of the branch must first be created as a branch object
VersionControl.CreateBranchObject(new BranchProperties(new ItemIdentifier(sourceServerPath)));
// Then you can create the branch
VersionControl.CreateBranch(sourceServerPath, targetServerPath, VersionSpec.Latest, null, checkinComment, null, null, null);
See also: