The attached error is seen when any field from Procurement - Receipts is being added to fields from Procurement - Purchase Order
@User_3512R - Make sure you include at least one fact / measure column from both the subject areas. If you do not want to display both the measures, then you can hide them by dragging and dropping them in Tooltip section.
Or
Create two different datasets - one for each subject area with the required dimensional attributes and facts / measures and then use these two datasets in creating the workbook.
Thank you, @Sumanth V -Oracle