The item 'XXXXX.xml' cannot be added to source control because the item has no source control mapping. If you choose one of the Ignore options the operation will continue without adding the file.

If you are facing such warning, as you try to synch work with VSTS, there can be two main reasons for this issue

1)     The object has not associated work item number. Ideally each piece of work that you want to synch with VSTS, should be having an associated Work Item number. In this case, you may login on VSTS and create a new work item and can associate your project with a work item.

2)    Other reason that I observed, was usability of an object into multiple projects. In this case the warning can come up if object is being used into multiple projects but you did not added it in project that you want to sync with VSTS. Rather you used in any project without adding it. In this case, to fix it, you can right click the object and add it to current project that you want to synch with VSTS.