According to this forum response from a Microsoft representative Multiple Lines of Text columns cannot be used in Calculated columns. The workaround that I always use for Calculated column limitations is to create a new field that will serve as your "fake" Calculated column (in your case probably a single line of text field) and then set the value using a workflow. If you do not want users to be able to manually edit the "fake" Calculated column then hide it from the new/edit forms using the SharePoint Manager 2013.
Note - This method is also useful for doing Calculated columns that use Lookup columns.