Please Follow steps for generate an order items category report 

Login on Back Office > from the left menu click on ‘Reports’  > click on ‘Sales’ tab  > select shop from ‘Select Store’ drop down > from the next drop down of select ‘Report type’ click on Details > now select ‘ORDER_ITEMS_CATEGORY’ option from the drop down list of ‘Sub Report Type’ Selection of ‘Select Employer’ is optional > select the date range from ‘From To Date’ click on ‘Apply’ button > as a last step click on ‘Submit’ button to view the report > report will be displayed on screen which you can download in excel format by clicking on ‘Export to File’ button at top right.