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  1. Create a PO with two lines.
  2. Then release the PO. It need authorization.
  3. So approval the PO.
  4. And cancle line 2.
  5. Then I cant’t cancle it.
  6. How to cancle the PO line with authorization?

Looking at the error, seems you need to use Purchase Order Change Order. 

RMB PO header > Change Order > Create.

Use the same path and view the created Change Order and Cancel the PO line and update the Change Order status to PO Updated.