You get this error when importing reports via the Import Reports feature in the Report Explorer:
Import file does not contain the DTools Report Definition File "ReportData.xml".
This is generally cause if you try to import an Excel Report or an "Order" Report via the Import->File feature in the Report Explorer:
These reports are not intended to be imported via this function. Only "standard" SI 2018/2016/2015/SIX Report archives (.dtr) and SI5.5 Report archives (.zip) are intended to imported via this function. For details on importing Excel or Order Reports, see Excel Reports and Import/Export Reports.